When converting between different FOVs with 0% MM, there would be 2 ways to do so. One is an approximation, where you're using the usual formula for FOV monitor match conversion but with a very small vertical resolution. Another way would be to calculate the limes of FOV -> 0 for this formula. In other words: If I want to calculate cm / 360° with a formula (can copy paste it on https://rechneronline.de/function-graphs/ if you like) for vertical match 100% I could use something like this: