I'm 5 batches into all grain brewing and this was a major problem for me. I just read Designing Great Beers by Ray Daniels and in it he lays out a method for calculating final gravity using the total gravity points. You do need to know the volume in the kettle to do the calculations. Basically you figure out your total gravity points for your target gravity and compare them to the total gravity points in the kettle.
So if you have a target gravity of 1.050 you take the gravity points, 50, and multiply it by your final volume of 5 gallons for 250 total gravity points.
Now after your mash take a gravity sample (make sure to correct the reading for temp or stick the sample in the fridge to cool off). Lets say you have 6.5 gallons in the kettle with a gravity of 1.040. The total gravity points would be 40 X 6.5 which equals 260 total points.
If you divide the total points in the kettle by your target volume you can calculate your final gravity: 260/5 = 52 or 1.052.
If you are below your target add malt extract based on gravity points per gallon...sorry I don't have the numbers handy but Kai gives a number in a previous post...
Each point*gallon you are missing in the kettle equals about 10g DME. If you are missing 5 points and have 6 gallon you need to add 300g DME. ( I hope this math is correct)
If you are over your target you will have more beer