You can download SimCalc directly from Xsimulator.net: https://www.xsimulator.net/community/marketplace/simcalc.184/
You can check out the mathematical assumptions behind SimCalc here: https://www.xsimulator.net/community/threads/simcalc-by-tim-mcguire.11277/page-2#post-140469
And adjust for screen scaling, if needed, as @Opast suggests: https://www.xsimulator.net/community/threads/simcalc-by-tim-mcguire.11277/page-4#post-256562