It depends, really, on how good your fermentation chiller is. With a lager you can get away with a lot skinnier cooler than you could with a hefeweizen.
With ale yeasts running under 65°F through primary I am comfortable filling right up to the weld line where the straight sides meet the curved top.
With a wheat yeast on aple juice I pitch the yeast on four gallons of liquid and then add another half gallon after a couple days.
If one way valve = spunding valve be sure to get a really good whirlpool so you limit the trub in the bottom of the keg. Use enough Irish moss for a ten gallon batch, cool your wort agressively, wait patiently after whirlpooling, I had real good luck waiting 25 minutes last time; and siphon carefully.
I have attempted two lagers, both were pitched on 4.5 gallons with temp control not as good as you are shooting for. Neither overflowed. Though both were dumped I don't think it was ferment volume that busted me.