Just to point two issues :
I'd suggest to take a look at the following threads on the subject of the cosine "method" and incline shooting.
Regarding those spreadsheets ( and some commercial programs too ) that uses the Pejsa's method, please be aware that they are based on a limited, basic approach and so they are far from being a full, complete implementation of the method.
They are based on the textbook's basic explanation, which in turn does not go to a full-length comprehensive explanation of the mathematical foundation for many cases not there described.
Especially when Velocity turns transonic and below (at long range ) or with Muzzle Velocity below 2000 fps coupled with low BCs.
In short, for typical rifle loads, results are acceptable, but no more than that.
IMHO, I'd say that the method developed by Prof. Pejsa is undoubtly the most accurate analytical method devised, especially for LRH. But again, a full complete implementation is not easy.