Yes as you says its down to the size of the gear v's the angle of the shafts, I found the 56t to be just about right, i had to file the edges of the braces just a little bit but its enough to keep the angle of the shafts not too steep. Plus 10 51 meant that there weren't enough teeth in contact, the 10 pinion was too small a gear with this power, it would rip the teeth off the spur. so 12/56 gave a better contact patch between the gears. Ideal would be something like 14/65 but the spur would be too massive to fit in.
