Theres no way really that you can be that accurate with the ppm by the way youve been checking, you really want to be aiming at a co2 range of about 25-40ppm to produce stable conditions.

This is the trouble with DIY, I banged my head against the DIY brickwall for a year before finally caving in and buying a pressurised kit, now life is great.

When you get your drop checkers, read up on Vaughn's threads about making a reference KH solution for it, that will make life a whole lot easier for you.