You might try a Nosler 115 partition with your IMR 4064. Nosler #4 lists 35.0gr of 4064 as Max! I personally have found that IMR 4350 is the best powder for the heavier bullets in the .257 Roberts. You should be able to work up some loads with 4350 and those Hornady 117gr. bullets that shoot tighter groups than those factory Hornady light-mag rounds.