I started using this free spreadsheet from BYO
and it works well. For this though you enter the target SG and it gives you an estimate for the grain bill based on your efficiency. I used it estimating a 68% efficiency and was 0.001 off. The web site has a good tutorial for how to use the spreadsheet.
I did some conventions of my old extract recipes with this spreadsheet and it came out to about 10lb of two row for 7lb extract.
But I think a lot has to do with the efficiency of the extraction process.