Don't worry.

I think the problem of how much vertical load the brace carries is more a function of the tightness of the joinery than of the actual geometry. If the brace is a little undersized, it will carry nothing until there is enough bending in the cross beam to load it and then start to share the load with the end support. (vice versa for an oversized brace).

Wood is very strong in compression parallel to the grain. (from the NDS tables, the value is 1500-2000 psi for most species). If you assume 1500 psi you'd only need 5.8 sq in. of cross section to support your entire load. A 3x5 brace would have almost 3 times this cross section. Because of the load sharing effect, I think even an undersized brace would never fail.