Originally posted by Zahmekoses
.... you would need more curves on a 55-digit-level to find for example a 49-digit-factor than on a 50-digit-level.
This is not true, actually, you need less curves, but each curve will take longer.

Try playing with the -v option in GMP-ECM and you'll find out that for every didgit level (45,46,47,48,...) there's an optimum B1 and B2. In practise however, it's easier to make 5 digit steps when choosing bounds. This makes it also easier to keep track on the curves done.

When i'm searching for smaller factors, i usually let the bounds increase each curve.