Beta-Styled Arch problem

[thesydneystrawboatsleeper]

Hello, I've been trying to make an arch similar to this old Goldrush one (the tunnel one on the top left), but I have no idea how to make it accurate enough. Any ideas or tricks for something like that? I'm kinda new to Hammer

 

[Skylark]

Make an arch or cylindar the size desired, radius looks like 64, then recreate the arc with brushes to fit within a square so it's neater and easier to work with. Duplicate to the other side.