Hi Gary,
The answer is to "profile" your printer for the paper and ink that you're using. This basically means using a colour spectrometer to look at a large chart of colours from your printer, compare them to a known value and write a profile to correct the differences.
Obviously, you don't sound like time is on your side. So what you're left with is a take a best guess approach. Lighten the shot on screen by the amount that you think it's too dark and have a try. It can take lots of your paper and ink so working on as small a version as you can is a good idea.