All measurements must use same units. Doesn't matter which units, as long as they're uniform. You can perform calculations using the diameter or the circumference, whichever is easier for you to measure.
I'm just a grad student, standing in front of a vinyl cutter, asking it to do my creative bidding. I'm just a maker, standing in front of an html file, enjoying the fact that automation makes our work easier.
Here's the Instructable that this page is intended for. Here's my website.