There is a rule of thumb for lowering PH but it depends on a reasonable alkalinity. The formula is for .2 PH drop:

Oz muriatic acid = .0008 * pool volume (gallons)

So for a .5 drop, you would need about 12 oz. Your alkalinity will resist the drop so you could probably put in more than that but I would start with 12 anyway. As Mike said, drop it to 7 and keep it there until the alk drops.