The problem is that each install is different. You can eithe rtry to figure out (on average) how large each record is and how much space the assocauited indexes will take up, or you can use some ballpark numbers. I have seen some spreadsheets that just use 3k/record as one guideline. For many installs, this will probably end up being high, but it's astart. Also, if you have a lot of attachemtns you are loading, those will skew the numbers quite quickly.