I'm guessing this is your first Tri build, so forgive me if I'm bloating.
Keep it simple.
Make your booms equal in length (Motor shaft to inner end. You have to account for the yaw mechanism on the tail boom) and space them 120 degrees apart. If you can, use 10mm square pinewood for booms
The geometric center is where the booms meet, if you imagine a line continuing from the inner boom end.
Try to keep your CoG close to this point, and design your LiPo-holder so you can shift the CoG later with the battery.
I just drew a line on the frame at this point, and check the pitch balance by holding my fingers there. For roll balance, I put one finger under the front center of the frame, and another finger under the tail boom.
Small variations in CoG are not critical to get a Tri airborne and resonably flyable.
Far more important are good ESC:s, Motors, flight controller, balanced props of right size for your build, and preferably no vibrations.