1/2 to 3/4, front and back. 46 years of transporting bikes and never had one fall. I've seen forks and shocks sag from too many overnights strapped down tight. When stopping for more than a meal and gas, loosen the straps. I've seen the front corner of a pickup bed used for a chock, and the differing angles resulted in bent forks when they were pulled down tight.
More important are the straps you use. Thise are the best I ever had, built in soft ties, spring loaded clips so the hooks don't come off the tiedown rings, sturdy ratchets:
These red ones are among the worst since the curved part of the hook does not wrap a full 180*, make sure the hooks are bent well around like the orange straps:
I found a bunch of these at a yard sale, cut the straps off the ratchets, cut the hooks off the long straps, and sewed the long straps to the ratchets for bundling straps, used for keeping things that are folded or rolled folded or rolled, things like tarps, tents, and sleeping bags.
I never use the type you push a lever with your thumb and pull the webbing through. I have seen many bikes fall because that type slips. They are no better than rubber bungie cords. Ratchets only for me.
