"Envelope calculation" refers to a rough or approximate estimation or calculation. It is not precise or detailed but serves as a simple way to get a general idea or understanding of something. Full definition
This is a back - of envelope calculation, the real way to calculate is with a finance calculator. (money.stackexchange.com)
This, right here, this back - of - the - envelope calculation suggests that it doesn't matter how popular or memorable or beloved he was. (sbnation.com)
A quick back - of - the - envelope calculation shows why. (treehugger.com)