How can I roughtly calculateTempered glass horizontal spans

I am contemplating replacing Lexan panels on a patio cover with tempered glass and need a reasonable starting point for the spans allowed by various thicknesses of the glass.  My spans are approx. 4ft between continuous frames and load lengths of approx. 50" and 80".  Any accessible calculators on line or knowledgeable suggestions welcomed. Wind zone is 120mph, no high exposure rating. Thanks

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