Inches times ounces before and behind the cg is what it's all about. If you have a metric scale and metric tape, grams and centimeters are a little easier to calculate. If you move a 2 oz servo back 20 inches, you have 40 oz.-in. added to tail moment, which must be made up in the nose.

I put a screw-eye or eye bolt in a hardpoint which I build in to the cg of my model (top for high wing, bottom for low wing), and hang it from the ceiling. This enormously simplifies playing around with cg, both fore & aft and lateral balance. Lateral balance can often be improved by mounting servos, batteries, other components up against one side or t'other of the fuselage.

Rather than add tailweight ballast, I am inclined in serious cases to chop the nose. This is a scary prospect unless you can calculate with confidence how far back you need to cut. After you have all the straightforward component placement taken care of, just hang everything from a cord and place lead ballast on tail hinge line until the model is level. Weigh the ballast, measure the distance from cg to tail hingeline, multiply weight times distance and that's how much moment you need to take out of the nose. Weigh your engine/prop/spinner/nosegear/firewall/tank power module. The next tricky part is to find the power module's cg. You can lay it all out on a little balsa tray, tie string across the four corners and nudge the hanging point where the strings cross until the tray hangs level. Straight down from the string gives you the power module cg. Move that power module cg back enough distance so that power module weight times distance moved will equal the moment of the tail ballast. Cut with confidence, re-glue the firewall and fly a balanced plane.

Sometimes it is as easy to move the wing forward as to move the engine back. The cg moves with the wing.