From what I've read here several times from our colleagues 75% is enough to keep the rim submerged and generally if you press the valve in withit set at 12 oclock you will get fluid back out but it will likely spit some air as well. That's where mine are right now with over 10 gallons. I don't know whose manual is correct but they obviously don't agree.
I think the reason that fluid comes out the valve stem is when it is at the bottom it gets filled with fluid then when the valve is it the top it still has some in it. What I do is shoot a little air in the valve before checking the tire pressure to blow the fluid out of the valve. If your valve is submerged when it is at the top this will not work.