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Oval Facet Problem - estimating final size before cutting

Hi … beginner here and I have a minor problem.

I have a piece of jewelry that has a fake oval (looks like brilliant cut) topaz and I would like to replace it with an actual gemstone. So I know the final dimension I want … but after looking at simple oval cuts I began to realize I have no idea how to judge how much rough (what size) I would need to get to a certain size stone that will fit the setting. With round, square, emerald, stepped, etc I can pretty much precut to size.

There must be some rule of thumb, etc … something simple I am missing? Like just add 20% to the edges to get to final shape?