Constructing an aircraft that the manufacturer has not included any suggested setting for the amount of right thrust/down thrust to be used when mounting the engine.

No thrust setting are included when the fuselage was constructed - fiberglass fuse.

Aircraft is a high wing, 88" wing span using a 45cc gas engine.

Appreciate any suggestions/information you can provide.

what airframe is it....

on our aircraft in that range (aerobatic)
0 degrees up/down thrust and 2.5 degrees right thrust.
then adjust to taste

Is it a flat bottomed wing or symmetrical? A flat bottom will climb on throttle. Might need more down thrust. An 88" high wing with a 45cc engine might not need any. Fly it and try it. Right thrust is used for a precise transition to a vertical upline. Some folks would rather use the rudder. More info, please!

Wing is flat-bottomed

The airframe is a PZL 104 WILGA. Used as a glider tow plane. Both in the model world and the full-scale world. Wing is flat-bottommed.

Hmmm.. that might need a scosh of down thrust... but I'm not sure.

Try it! If the plane zooms up when you advance the throttle from a level flight, then it needs downthrust. Add downthrust until there is no zooming with throttle. If the engine is bolted to an engine mount, add the thrust washers behind the mount rather than under the engine lugs. It is best to have the engine lugs contact the mount all along their length and for the mounting surfaces to be flat, parallel and in the same plane.

Regards,

Richard

The Wilga should be trimmed while towing the glider.
Leave downthrust alone at first. It takes away from pulling ability. You are not after 3D neutral flight, and the engine must pull hard at low speeds.
If I were to design it, I would start out at 0° (neutral down), and about 2° of right thrust to account for prop wash.
If in the tow you must trim the plane down, add downthrust as needed.