Exactly what I do.I do a slightly different way.
First put some marker or layout dye on the pivot to mark the length I am aiming for. Then take some scrap steel and drill a hole to match the pivot size. This then lets me lay the scarp steel against the flat platten pretty square and put pressure on the pivot to force it into the spinning belt. Be careful here - the pivot will get hot quick so dip in water regularly. Once you are getting close to your scribed line I move to a pivot lap and do it by hand with my sand paper layed on top of a flat surface (I use some granite). The pivot lap will give you a nice square end and also allows you to dial in an exact length easily.