If your pH meter isn't designed to go into the soil then it's not going to work properly that way. If it's one of those cheap moisture meters, that has a switch you can flip to test the pH, those are so inaccurate that they are useless. The amount of pH up/down needed is dependent on the buffering capabilities of the water you are using. Always start with a tiny amount, stir it in and test, then repeat until you get where you need to. After a while you will just get used to how much you need to add and will just need to test it a second time to verify you got it where you need it.