I am not sure how you arrived at the 60% number. I agree that it is somewhere in that range, but don't see where the hard number comes from.
If you use Kai's spreadsheet, you'll see that using 10# of grain to make about 6.5 gallons of wort, pre-boil, from equal runnings, should yield about 63% of the total available sugar. I rounded that to 60%. My experience has been that ~60%-63% is a realistic real world figure as well. I was also taught that 60% of the available sugar per running was a good way to calculate the expected gravity for partigyle mashes.
Kai's spreadsheet calculates that a similar no-sparge wort, could yield 77% efficiency. I reliably get about 70-75%.
Essentially, the ~60% and ~75% comes from the percent of the volume of water you drain from a tun relative to that added to the tun (i.e., minus grain absorption), with a correction made for the volume added by the dissolved sugar.
Open Office is a free alternative to Excell. I think that works with Kai's spreadsheet.