I'm definitely no expert, but here's my 2 cents anyway:
Mash pH is where you want to use the meter. Pick an acid (I use citric acid, but most brewers prefer phosphoric, and some use lactic). Or use acid malt. Use software tools or just kentucky windage and try to hit 5.2pH. If you miss, sadly you'll have to wait until next mash to correct since I don't think it's possible to correct mid-mash.
Measuring pH is tricky because pH is very temperature-dependent. I take some mash wort, put it in a tiny closed tuperware container, and set it in cold water to cool it off. When it gets to room temp, I measure pH. I suppose you could come up with an offset of some sort to correct a "hot" measurement, but I don't trust that.
I use 10g of citric acid in a 10g batch.