You have answered your own question right there. Think Center of Gravity that is what is important. but left out where it balances?? The firewall to thrust washer info is normally important for airplanes with cowlings.

If you have no info one what the 'CG' range should be or the true identity of the airplane may be in doubt then balance at the quarter chord (25%) simply divide the distance from the leading edge to the trailing edge and divide by four, that is the distance back from the leading edge to target for your center of gravity. That is a safe place to start at and can be fine tuned in later flights if desired.

Your are correct most sticks are set up with zero - zero thrust lines. the clamping type engine mount while not my favorite, they can work fine.

John