ARF mfgs don't always get their tank hole positioned so you do need to focus on the mounting holes. When they form a square, as they do on your firewall, MinnFlyers connecting lines give you the perfect center of the engine mount.

It's always best to pretty much ignore the tank hole and assume you might have to "move" it some. The one on your firewall isn't actually centered relative to their original mount. If you followed it's location, one might think it was positioned for up thrust and no side thrust if you didn't look at the motor mount holes.

One of the 1st ARFs I assembled when I got back into model planes was this Phoenix Sukhoi. It's tank hole was located for upthrust and the instructions didn't explain anything. They had a picture showing the tank hole centered in the motor mount. The firewall had downthrust and right thrust built in. At the time I couldn't believe a model mfg would get such an important detail so screwed up. I saw 2 of them later and one had the spinner off center in the cowl. The other had the cowl installed at an angle.