I always use 6 with normal EC values and 6.5 using high EC values. That's because when you use a EC of say 2.4 the soil uses it's chalk asa EC buffer aswell so it doesn't have much room left (chemicaly speaking) for selfregulation of the soil's acidity.

When I'm growing hydro I'm using a ph of 5.6 in veg and 5.8 whiles blossoming.