For PPL stuff I find that the following works for me:
(1) Work out the distances for MAUW, zero wind, applying all the usual fudge factors. One for tarmac, one for grass.
(2) Use those for each flight.
(In other words, I've done the calculations once, ever.)
If a runway comes out looking too short I simply don't use it. I've never felt tempted to re-do the sums in order to try to shave a few percent off because there's a headwind, or I'm not fully loading the aircraft, or it's a cold day, or I know they've just cut the grass (honest guv). I personally would regard any such temptation as a danger sign!