Thanks for the reply, AlexC.,
If you have any "stand" like a C-Stand, a Microphone stand, or anything that can extend to that height where the project needs to be, would be a great help. Set up the top part where you want the projector's lens to be. From that point, the projection wall must be fully visible. To have that stand there helps to get fast to the final position. (They certainly have a ladder that might work, but typically that is moved when mounting 😉
If my exploration is not off, then a 45º is the horizontal field of view, which is easy to create by folding a piece of paper (square, fold diagonal); this will be the fastest guide to see. To see means here that you can see the projected wall ultimately.
Many iPhone cinematography apps allow you to work as a director's viewfinder; with a full-frame camera, a standard 50mm landscape mode will give you a safe estimation (a 40mm, like the Sigma Art or Canon's Pancake lens, is closer, full frame as well.)
My best wishes for your project.