Use `pgfmathsetlength`, and max progress bars out at 1

[kno10]

Use pgfmathsetlength, and max progress bars out at 1. This should also avoid the problem of a too long line on the first compile.

[kno10]

This should resolve #274; but this pull may still have a syntax error - I edited this in github. Also you may want to squash the two commits when merging.