I like to balance, and weigh, the plane's frame before installing that tank, radio, engine, push rods, etc.

Then I measure the weight of the individual components, and measure their prospective location on the plane.

Then I make a little spread sheet to estimate where the CG is going to fall. The spread sheet looks like this:

Weight (oz) Component CG location (in) Mass Moment (oz-in)
bare plane xxx yyy xxx * yyy
engine xxx yyy xxx * yyy
servo 1
servo 2
battery
receiver
tank
spinner
prop
pushrod 1
pushrod 2
etc
balance
-------- --------------------------
Totals W oz XY oz-in

Overall CG location = XY / W (in)
To measure the component locations you pick a single point on the plane and measure
everything relative to that point. Components behind your reference have positive
location values, in front they have negative values. The overall location is relative
to your reference.

The simplest reference point is the leading edge of the wing since the desired CG on the plan will be referenced to that location. But, I prefer to use the firewall simply because it is a good place to hook a tape measure.

The last entry in the list above, !QUOT!balance!QUOT!, is the balance weight. Enter zero to begin with.
If the overall CG is no good, then you either move some components around on the spread
sheet to get it to work out, or you add some balance to the mix.

I've done this for a UCD46 because I read they often come out very tail heavy. After sorting it all out with the spread sheet, the actual CG came out nearly exactly were I estimated it would fall. I choose moved the rudder servo up from the tail to under the wing, shifted the throttle servo forward a bit, choose a battery location under the tank, and selected a more powerful and heavier engine - all based on the spread sheet.

I've done the same for GP Tracer with similar results. I first tried to balance it by simple putting everything in and seeing what happened. It was way out of wack and I had to pull the gear out. Then I made the spread sheet and decided to move a servo back to the tail, and put in a small compartment for the receiver and battery behind the wing. It I had used the spread sheet to begin with it would have been simpler.

If you don't care too much about the total weight it is not worth the trouble, however.