Arch Voussoir Calculations

[QuinB]

Here's hoping this is being posted to the correct section!


I've got a real problem trying to duplicate this arch style.

I need to match the angles of the voussoirs over this window; I'd thought it was a plain segmental arch, but the outline seems to be elliptical, not circular; that's all very well because I can find detail on calculating templates for either, but the patterns  doesn't match Real Life.

Can anybody help me sort out what's going on?