Here is my method for getting lugs lined up correctly (spaced evenly) on the shell. It isn't very scientific, but it has worked great for me each time I have done it. I have done it on lower end drums to add lugs, such as if the original rack toms had five lugs, I would make them 6, floor tom had 6, I would make them 8 etc.
I made a modification, since I have never had to start from scratch on a drum or particular side with no existing holes. I added Step 1 for your situation, I usually start mine on Step 2. Note: For your situation, the first paragraph of Step 2 will not apply. After step 1, follow the "From a 5 lug to 6 lug" section in Step 2. Also, using a flat, "fabric" type of tape measure is advised, like the kind a tailor would use for measuring your waist or neck.
STEP 1: Assuming you have the bearing edges cut already (which you'll need to do on a concert tom), take the hoop and head off the batter side and take off one lug (it doesn't matter which one). Then, put the head and the hoop back on, and install the tension rods, getting them just finger tight so the hoop won't move or shift. Where the lug is is missing (that you took off), put a long tension rod, or even a long screwdriver through the hoop "ear" just like you would be putting the tension rod on the drum normally, except this time of course there is no lug for it to go into. Make sure the long tension rod or screwdriver goes down far enough to where the lug would need to go on the reso side, so you can mark on the shell with a pencil or Sharpie where the tension rod lies. I find it helpful to push the tension rod/screwdriver inward towards the shell, and mark it's position there, where there is a little less variance. Make a decently long vertical mark on the shell, an inch or two long, approximately where the lug is going to be. NOTE: You can always put painter's tape on the wrap near the approximate area where the lugs are going to be if you don't want to unnecessarily mark the wrap, although Sharpie comes off pretty easily with lighter fluid or goof off. After you get the vertical alignment marked by using the method above, take the hoop and head back off the batter side, and measure how far down the center of the top lug hole (closest to bearing edge) is from the bearing edge. Once you have marked that (by using a horizontal mark, again a few inches long and which will "intersect" your previous marking of course), measure the lug hole spacing from the center of the top hole, to the center of the bottom hole, and mark the shell accordingly, again using a horizontal line a few inches long. Where your vertical and horizontal lines meet (aka the exact spot of where the holes need to be drilled), I make those "+ signs" (if you will ) a little bolder and easier to see. It helps when you are drilling to be precise. DOUBLE CHECK YOUR MEASUREMENTS! Drill those two holes per your measurements and the correct size holes for your lugs.
Step 2: This is where I usually start. This is the starting point for adding additional lugs. If you're going from 5 lugs to 6, fill in all existing lug holes on both sides except for one (doesn't matter which one). If you are going from 6 lugs to 8 (or 8 lugs to 10), then fill in the existing lug holes except for 2 on each side. The two unfilled should be 180 degrees from each other (On 6, 8 and 10 lug configurations, they all have a lug that is opposite, or 180 degrees from it). When filling in holes and/or leaving them unfilled, be aware of bracket holes or strainer holes etc. Changing the number of lugs will make the existing bracket and strainer etc. holes off center in the new configuration. Same for breather holes. I usually fill in the breather holes and just drill new ones. Same for bracket holes, I usually fill in and just drill new (or use a suspension mount). If you want to keep existing bracket or strainer etc, holes, you'll have to plan the position of your lugs accordingly.
From a 5 lug to a 6 lug (or to continue from Step 1 above):
Install the lug into the only existing hole. Then place a hoop (that has the "new" number of lugs, aka a 6 lug hoop) and head on the drum, and install a tension rod into the lug and get it about finger tight, just so the hoop won't move or shift. Then take your normal tension rod that you've been using on the drum and place it in to the hoop ear that is 180 degrees from your lone lug. Mark on the shell where it lies just like you did in Step 1 (again, I usually push it inwards so the tension rod is touching the shell so it's easier to mark). Once you have it marked, take off the hoop and head and measure the distance between the center of the top lug hole (closest to the bearing edge) distance from the bearing edge, again just like Step 1. Once that is marked, measure the lug hole spacing center to center, and mark that on the shell as well. After DOUBLE CHECKING YOUR MEASUREMENTS, drill those lug holes according to your measurements and the correct hole size for your lugs. Then, install a lug in your newly drilled holes, and the put the hoop and head back on, with two tension rods. The two installed lugs (180 degrees from each other) should now line up perfectly with the "ears" on the hoop.
NOTE: I always "spot check" myself after this step is done. If it's correct and looks good, and the two lugs that are 180 degrees apart line up perfectly with the hoop ears, then everything else will too going forward.
From a 6 (or 8) lug to a 8 (or 10) lug, and to continue your 5 to 6 lug from above:
Install your two lugs that are 180 degrees apart (if not already). Place the hoop and head on the shell and insert two tension rods into your two lugs that are 180 degrees apart. Get them decently tight, again so the hoop won't move or shift. Insert the rest of your normal tension rods through the empty hoop "ears". Make all your markings and measurements where each tension rod is, just like you did above. Once you've done all your drilling, reinstall all your lugs and then put the hoop and head back on. All the hoop ears should line up perfectly with the lugs below. I install the tension rods and tune it up just to be sure everything is correct.
Again, this isn't very scientific, but it has always worked perfectly for me, and I've done it at least 6 or 7 times.
I hope this helps,
V
