It depends on the situation.
Generally speaking, setting your flash to HSS and dialing down the FEC to -1.0 or below, or thereabouts, can get you started doing fill flash. You'll soon find out whether the flash intensity is enough or not by your results.
I've found that at times FEC needs to be at 0 or even +0.7 to get the results I want - again, depends on the ambient light, subject and general setting...
Probably best to read up on it online, like from:
www.profotos.com/education/promag/.../fill_flash/index.shtml Shutterbug: Using Fill Flash Fill-Flash