I've found that My BobCat XL thrust line is correct when the engine thrustline matches the angle of the top surface of the wing skin adjacent to the engine.
If your XL climbs when transitioning from cruise to full thrust, you need to increase the downthrust. Raise (shim) the front of the engine upward slightly to add downthrust.
I use a spirit level set on the top of the wing centerbox right beside the engine. If it is easier for you, level the airplane so that this angle reads zero with the level in place.
When satisfied that the wing skin by the engine reading is zero degrees, move the level to the engine case. It's okay if it sits on top of the engine mount straps (but not the nut and bolt flanges). Adjust the engine in order to attain the same zero reading. Make sure that the centerline of the engine is on the centerline of the tee flanges that screw to the engine mounting angles.
This should be a good starting point. You can readjust the thrust angle from that point to get it right on the money. It shouldn't take much.
I have set-up several BobCats this way, and they have all been in the ballpark.
Each of the BobCat engine mounts are designed with the proper amount of downthrust built in. You should be pretty close as installed.

After determining the downthrust angle, you can check it for side thrust by doing a large, smooth loop. If it comes out off heading to the right, you need a slight amount of left thrust, and vice-versa. Just make sure that you laterally balance the airplane by adding weight as required to make each main wing panel the same weight. Weigh the panels on a small balance scale, and add lead to the aileron servo pocket of the lighter one until the panels are the same weight.

Check the CG to verify that the airplane balances on the 1/16" holes on the bottom of the booms, with a full UAT (No other fuel on board) and the landing gear down.

Hope this helps.

Harley Condra
BVM REP
Team JetCat