For a .46 2 stroke I use 10x7 or 11x5 or 11x6. (Bushing .46 would use a little less)
11x7 used to be considered a prop for .60 size engines. Over-propping the .46 will lessen it's power by getting it out of it's peak torque (not necessarily peak RPM) band.
I have a Magnum .91 RFS too. I get about 9700 (richened) RPM on a 14x6 APC - about perfect for that engine - using 15% nitro Omega fuel.
A tach is useful for getting data, but isn't really necessary for a 2 stroke.
Let us know what happens.