Measuring sheet into semispheres

[AurumArcanum]

Forumula of disk size for dome making

Today I wanted to create a dome that would have an 8mm inside diameter and a 9mm outside diameter (.5mm sheet). I had absolutely no idea where to start to estimate the right size disk for starting - and hence I have two very sore spots on my fingers from so much sanding down! DOH! are there any formulas for this? or good rules of thumb even? I am mathematically challenged so if this is obvious I do apologize Thank you for any help! Lucy

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