From what I remember the important parameter is not voltage itself (it depends on many difficult to control factors, including resistance of the solution and object geometry) but current density (A/m2). Sorry, no idea what is the best value, it is something that has to be chosen experimentally.
I would go for something like: increase the voltage till you see gas starting to form up, measure the current, then decrease the voltage to halve the current. Not guaranteed to be optimal, but at least should be in a reasonable range. The higher the current the faster the plating, but slow deposition produces better quality surfaces, so if the surface looks grey or flaky, try to further reduce the current density.
Just an (a bit educated) guess.