I'm sure others with more experience on bands will chime in but I think .6 may have too much horsepower for 5/16 if you use much taper at all. If those were my only options I would start with a straight cut 1/2" or 9/16" and go from there. I would also use as small a pouch as I could get by with to keep the hand slap down.
My personal experience is to not overthink the band thickness to taper ratio. All my bands regardless of latex thickness use the same taper per size of ammo that I shoot. The 18:12 is a great size for 5/16 ammo that will produce good speed. In fact, thinner latex usually means faster speed. When you cut the band, just give yourself a little bit of extra length if you are afraid it is going to be too strong for the ammo. It is much easier to custom tailor the band set by adding or shortening the fork tie on point.
But while you are experimenting and learning, make both, the tapered band and a straight band out of the same material and length. Shoot them both and note the difference in speed. Tapers are all about speed and acceleration.
My personal favorite taper is 20:15. I shoot this with 3/8" and 5/16" steel for all latex .7mm to .45mm. Band length is the variable that can tweak the hand slap factor if that is a problem.