e30 325is exhaust modification

[Beef Supreme]

I'm thinking of replacing from just ahead of the cat back with a single 2.25" system. My question is this. I'd like to know if anyone has figured the optimal location to merge the secondaries into a single pipe, "a la" tri-y. Does anyone have a tri-y header?? Care to measure?? I'm willing to do the math to figure out where to put the junction I just have no idea where to find the formulas. The engine is essentially stock btw - so far just a miller MAF conversion and chip. Thanks. A stock system is very expensive with that proprietary cat and stuff so please don't be suggesting that ; )