Victor there are a lot of unknowns here. In a lot of wood cutting a 5/16th in one pass is very doable. Wood such as pine, western cedar and others. Very hard wood might take two cuts at the most. Also a lot depends on what router bit you are using and the size and quality of the bit. A 1/8th bit is very fragile while a 1/2" bit will be stronger. Maybe some pictures would also be helpful. The more information we have the better answers you will get.
Don in Murfreesboro,Tn.
Measure once cut twice and it's still to short.