I'm shooting 340s out of several bows, a 49lb widow and a 46lb Morrison ILF and a 53lb cut to center pronghorn. I draw 29 to 30" depending on the grip. The shortest 340 set that I have is 31". I have 300 gr. up front giving me about 22% Foc. I also have some K2s(340s) that have 350 up front, as they are heavier to begin with needing more weight to tilt forward. I like to keep my arrows long so I will go up to the next size before I cut them down to much. I get better flight and paralax with the heavy point and insert hanging past the rest and off the back of the bow, giving it more leverage to bend the arrow coming out of the bow. I also have some 300s that are just a tad stiffer at full length than a 31 1/4 340. That is with 300 up front as well, on a beman ICS elete.
The other day I was shooting a 32 1/2 340 off of my old bear takedown with 53lb #3 limbs. That is with a dacron string at 30 draw for sure with a 2r grip. Hope this info helps.