The percentage of shots for (SF%) is a ratio of the SF to the shots pertaining to a given player (SF + SA). In other words, SF% = SF/(SF + SA)
As you can see in the second spreadsheet, Brian Boyle had 3 shots for (SF) and 3 shots against (SA) during the 6.9 minutes he was on the ice for the whole game. 50% of all shots involving Brian Boyle were his Shots for [3/(3+3) = 0.50].
Is 50% a good percent? I dunno, let's compare it to the average. We do this by comparing Boyle's SF% to the rest of his teammates SF%, resulting in Relative (to Rangers players) shooting for percentage, or called "Rel SF%" in the spreadsheet.
I see Brian is 13.6% less than the average, making the average 63.6% (Brian's 50% + 13.5%)