Without a multi probe egt, all you can do is slowly reduce mixture until you either hear or see a drop in RPM, then either leave it there or enrich a tad for smooth operation.
The FF figures in the POH are based on correct leaning technique so you have to follow them. the biggest problem I see with leaning methods, is pilots tend to rush the process and get a false peak when using a single egt guage, then not re-leaning after a change of altitude, to lean properly, you have to do it slowly.