It really depends on the paint and the pot. There is no magic formula, because even the same exact paint will vary pot to pot unfortunately. Most of the
GW "Base" paints are quite thick and need more water as a general rule.
You can paint with mixtures as viscus as tinted water, but it requires a little more skill and practice, particularly on how much you load up on the brush and/or wipe off before applying to the model.
It does help to have a pallet. For going super cheap -- I use these: