can someone explain to me how double washing works?

You generally get the minimum MP for leveling up minus 2 when you spend AP, same as HP, afaik. (except on warriors who used to get another 10 less for AP cause the skill was +20 on AP, +30 on levelups)

thing is, each ap i spend on HP is 51~ hp gained, at the cost of 4 MP, to be able to beat that with double wash, i'd need to have a gain of 12 mp per ap, and that's 3 reset tickets instead of just one.

however, the difference when leveling+wash with 200+ int is noticeable, but dear god if it isn't expensive. i think imma keep doing the old kind of wash.