MY 2 CENTS WORTH.
As a rough guideline, consider the following for flat putts above 10 ft,
slow : stated distance + 1 forth of that dist,
standard : stated distance + 1 sixth of that dist,
fast : stated distance,
very fast : stated distance - 1 sixth of that dist,
tournament : stated distance - 1 forth of that dist.
Clearly, any upward or downward slope (calling for this approx. formula : distance number in feet plus or minus height number in inches = putting distance number in feet), not to mention side slants, must also be factored in.
I hope that will help :-)