Don't know of any online applications. They'd need for you to input some numbers, and if you get those numbers you can scribble them into an answer on paper about as fast as you can bring up an online app and type them in.

I don't remember when it was in school they taught the moment arm deal. That's what you use. It'll take more words here to explain, and if I try, it'll sound more complicated than it is.

Let me make a suggestion... or two.
You already have decided to move the servos. And you already know you need to. So move 'em and check to see how far that moved the CG. Try not to break out in tears in front of a woman. The most important suggestion is to accurately mark your starting CG. Unless you do that, you won't have a prayer of even guessing what effect the moves made. Do that and you'll have a good idea how much more weight MIGHT be needed.

I did just that and discovered I could move my battery pack back into the fuselage and get the CG I wanted. Cutting the ultracote out of the bottom of that airplane provided access through a semi-permanent hatch. Simple and quick. I built a balsa box for the battery and glued it in. Easier to do than trying to put braces in that fit the space and the battery. Ironed on matching ultracote and "closed the hatch permanently".... or until I wanted to get at the battery.

Working out the moments really isn't hard, btw.