You've lengthened the tail and a few other things, right? The start location for a CG is a function of the size, location and shape of the horizontal tail (the part of the airplane "responsible" for maintaining pitch) and the size and shape of the wing. Measure those features, plug the numbers in and within seconds the application will spit out your numbers http://www.geistware.com/rcmodeling/cg_super_calc.htm

It's quick and easy and can give you a range for the CG that's safe and reliable and considers what the flying world uses day in and day out. Plug in 5% static margin and run the application. Note the CG, then plug in 20% and run again. You'll have a range that'll show you where to get the CG plus the size of the range will indicate how sensitive your plane is to CG. An amazing amount of excellent info for almost zero work.