.
There are diagrams and estimated curve radii for all the common sweeps in most carving books and many manufacturers publish estimated curves and sizes for their range.
This book by Chris Pye is excellent and deals with exactly what you are asking about.
http://www.amazon.co.uk/Lettercarving-W ... 501&sr=1-4
One thing to be aware is that the radii versus sweep number differs very slightly between the traditional British (London) pattern numberings and modern continental sweep numbers. The numbers don’t exactly follow the same sweep d graduations. There’s not much in it in ordinary carving, but it may make a noticeable difference in lettering. You probably know that the curve increase with the number. A number 1 is the shallowest, almost flat, whilst at the extreme, Number 9, is a semi-circle in the London system.
It’s not easy to reconcile gouge sweeps to their size going up the scale and different manufacturers seeps vary slightly. For example, a quarter inch No: 2 (shallow curve) is a greater circle diameter than a quarter inch No: 4 (steeper curve).
One practical tip that helped me is for small lettering jobs (just a few words or numbers), is to type it out on a computer, space and size it to your liking and use the print as a template glued on the wood.
Another is to assess what tool-sweeps you already have. (I think that this method is in Chris Pye’s book).
Get a piece of good carving wood. Line all your curves up on the bench, then with the gouge vertical, gently scribe it’s line by rotating it gently and allowing it to follow its own radius. That way you will reveal the curve of each gouge as a circle and use it to select gouges for each curve. By doing this for all the gouges in your collection you will have a reference board that is specific for your tools.
Hope this helps.
.