The recipe I use calls for 10% PMA in ethanol. So I just weigh some amount into a Erlenmeyer flask and top it up with 96% ethanol (eg. 5.6g of PMA -> 56g of final stain). While developing plates I dip them briefly, let them drain and then heat the hell out of them with a heat-gun. The fresh stain is yellow and can be used for some time (couple of weeks). Then it goes green and somewhat is less effective. I've never tried adding sulfuric acid, but maybe doing so will not require such a high temperature (prolonged heating at high temp. sort of darkens the whole plate).