You'll want to use PoolMath to figure out how much you should add to get from one reading to another reading.
Click the link at the top of the site here, enter your pool's size at the top, go to the bottom and select TroubleFreePool method, select your pool finish type (vinyl, plaster, etc), and select the type of chlorination you're using. Once you've done that, you can enter your current Ph, then your target Ph, and it will tell you the exact amount of Muriatic Acid to add.