Not directly helpful, but I'd be tempted to go to 100gn in 260 to keep the velocity up. I run 120gn TTSX in my 6.5-284 as there's plenty of velocity to play with. I'm currently working up a load for 95gn GS Custom (lead free) for my 260. N530 and settling on a load around 3,150fps.
Back to your original question though, Vhit lists the following starting and max loads for 120gn TTSX in 260 with N540 (note - they list N540, N140 and N150 but not N160. I found N160 to be too slow in my 260 when running 120gn Nosler BT):
N540 - start 34.1gr, velocity 2457fps MAX 38.7gr, velocity 2780fps
General guidance is start with a 50thou jump for TTSX and go up from there in 25thou increments. I think I've got about 90thou jump in my 6.5-284 from memory.