Are you shooting smokeless or Black powder in this?
For smokeless, I like having bullets be .002" larger than groove diameter, in general. For rifles with really huge throats (a certain Hungarian straight-pull of mine in 8x56 comes to mind), I like a bullet even bigger.
I refuse to shoot Black any more, but when I did, I initially tried bullets about .002" over groove diameter. Frequently, Black powder was expected to exert such a shock to the base of a soft, undersize pure Lead bullet, that it would upset to fill the grooves. I used to have a pair of .43 Spanish Remington rolling blocks that had .446" groove diameters, but would not chamber a cartridge with a bullet larger than .441", even in ancient Berdan-primed brass.
Rifle barrels are frequently mighty particular about their likes and dislikes. Can I suggest you buy some commercially cast bullets in popular designs, in several different sizes, and see what the rifle likes?
There is a Single Shot Rifle Association, with a discussion forum. You might want to find it and ask for experience with your rifle make and model there.