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Is there any calculation that will tell me how big to make a piece of glass to fit in a mold?

I have a mold on its way to me, 9.125" circle, probably 1.5" - 2" deep in the center. It looks like there's a half inch flat rim around it. How big around should my fused piece be to slump into it without either having glass drop off outside the rim, or pulling in off the rim entirely? Is there any formula or calculation, or is it just a "trial and waste" kind of thing?

Just cut your glass the size of the mold.......or you can go 1/2" larger on a mold that is deeper as the glass will move in as it drops. You sound like you may be a new fuser. It is probably best as a beginner to just go with the size of your molds until you see your results. Take notes!

Not a new fuser, but definitely a new slumper!! Thanks for good advice! My square candle dish mold I've pretty much perfected. I decided to get something round just for some variety, and deeper because I wanted to start to give myself more challenges/opportunities to learn. I think as you said I will do my first test with exactly the size of the mold, and then do some measuring.