my Revolver arrived via courier... super stoked!... it has a big tank in there... not use to it... 420cc... this tells me that I might not need to cut out servo holders in the rear... cause I could just compensate the weight by adding smaller tank... or how does one check CG out... full tank in consideration or empty?

It should be balanced with the tank empty

Ken

there goes my theory then... cause I have seen guys on youtube with 115 Saitos in there... but they all are still lighter than my Rossi...

Servos won't make a huge difference balancing unless they are mounted in the tail as some are.
The battery is the heaviest item that can be moved around.
I will balance a plane with the battery out and lay it on the plane in various locations to see where it needs to go.
You can even add a hatch to access a custom made battery compartment.
Balance with the tank empty.

420 cubic centimeter = 14.201 889 535 ounce [US, liquid] so it's not all that big of a tank. A 16 ounce tank isn't uncommon for a .61 engine.

thats the plan, cause I can counter with 45g - 135g there...

Lay the parts out on the plane while balancing before deciding where everything goes.
Remember that the fulcrum will be at the recommended center of gravity and the farther away from that point the less weight you will need to balance. You may find you will only need 1/2 oz or less in the very end of the tail and can mount the servos in the original position. In other words just because you have a heavy engine in the front does not mean you will need alot of weight in the rear.
In fact the heavy engine may even be helping you. I have found that most models that I have had to add any weight to for balancing have been tail heavy to start with.

yeah that was the plan... I was planning on doing so... yeah I know that the further away the less weight... servo weighs 45g each, which might be too much weight all ready...