Different types of speakers = different distances. In my experience panel type speakers will work best at 5+feet where you can expect greater clarity due to increased time in the signal bounce from the wall behind the speakers. Ironically, due to their dispersion pattern you can usually place them closer to the side walls than box speakers (which in turn you can place closer to the wall behind them than the panels without losing as much clarity). It is hugely helpful to understand the existence of, and location of, bass nodes (using a SPL meter and source of LF sound) so you can avoid placing your speakers in a bass node (and your listening chair as well).
Oh, BTW, measure from the speakers front baffle as suggested above.
Treating the wall behind the speakers appropriately can reduce the need somewhat if needed.