i would atleast use a 100grain for starters... 100 grains seem to be majoc for me, but then again I usually add weights to the inserts to make my own bolts. I like my bolts anywhere from 375-405grains, normally 390's is perfect for me. I like to get a weight thats not to heavy, nor to light

but try a 100grain bhead and see how it does. The lighter 85 one might make the arrow less stable... you might not see any difference from 0-15 yards, but after that you might see some zigzag

goodluck and happy shooting