It might be that changing monitor size in combination with stretched resolution has a bug I see, as it assumes you have a 24" 4:3 monitor and are converting to a 24.5" 16:9.
0.55 is correct, if you do like this example to change the horizontal FOV to 90 while keeping your 360 distance you'll get 0.55:
https://www.mouse-sensitivity.com/?share=1a41515e93266b224305e65fee159963
I'll fix this in the next update!