5/16" for a 2" wide aileron is just about 9 degrees of travel. For my biplanes, I use about 12 degrees. However, if your aileron is top hinged, it sounds like you should have no trouble getting all the up travel that your linkage is capable of providing and that your problem is getting enough down aileron. For down aileron, 9 deg is probably fine. Most bi-planes need ailerons differential where the up aileron travels more than the down aileron does. 12 deg up and 9 deg down is probably about right.