Using Old Glasses As Differentials

Ever since I’ve heard of using under corrected glasses for close-up, I immediately opened my drawer and decided to use my old glasses (maybe a bad idea). They blur around 60 cm for both eyes. I don’t know what the prescription is but if it blurs around 60 cm, can I put that in the diopter calculator and find out or does it not work like that? Should I get proper differentials or can I just use these glasses?

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Thank you!