I do use a rail square - the Insta RailSquare - which I've had for about 3-1/2 years. Brilliant for site work
Current kit:
Festool TS55 , Festool TS75, Makita DSP600 (2 x 18 volt), Hilti WSC 265
Rails:
Festool 800mm, 1400mm (3 no), 3000mm
Makita 1000mm, 1500mm (3 no)
Insta RailGuide...