Jim-
Here's a link to start your estimating:
How To Choose The Best Shotgun Barrel LengthSee especially the second section of the article, "Shot Velocity". An estimate in moving from 26" to 20" would be a loss 45 fps with magnum loads. The velocity loss per inch is not a constant, and there are a lot of "it depends" cautions connected with making an estimate.
Are you discarding a fixed choke? Planning on having the shortened barrel re-threaded for your choke tubes? Just curious.
--Bob