I have an Excel spreadsheet that I use for calculating CG, it was given to me by an old timer friend of mine. You level the airplane, weigh it at all three gear, add in the meausrements to the datum and it does the rest. The nice thing about it is you can pick a point on the airplane, add or remove weight and it will calculate where the CG will move to. As long as you have a scale it's basically free and you know exactly what adding or removing weight from any place on the plane will do to your CG.