Happens that I asked that question before, perhaps by PM. Ty Marcucci said 4.25 oz was enough, but as always, it's easier to put less in than the maximum. Brett wrote here that the way to get a consistent amount in the tank is to fill it and then take out some, using a smaller syringe than folks use for filling (smaller diameter = more accurate).
Personally, I use a "mustard pump" (they'll blow up a combat bladder), and at least the ones I've gotten from TAP Plastics pump EXACTLY 1 oz per full stroke...but you do have to "prime the pump" at the start of each flying session. I made up a double loop thingy from coat hangar wire to hang the 1 oz syringe from the pump/jug, which I use to pull out excess fuel.
I am fussy about keeping my fuel clean, and really cringe to see guys open a gallon jug, suck some fuel into their 5 oz syringe, and lay it in the grass or on the pavement. A lot of those guys have changed to using electrons....
Steve