I remember learning that paper was usually quite expensive before the Industrial Revolutions, so were mental maths encouraged to make up for the costs of it?
Don't forget about other non-paper tools that they would have had.
There's a comment here by /u/sunagainstgold/ talking about wax tablets, used as a temporary writing surface in medieval western Europe:
Here's /u/Tealwisp/ making their own wax tablet:
https://www.reddit.com/r/AskHistorians/comments/tx06g/wax_tablets/
Here's /u/Bodark43/ talking about wooden tally sticks, used to record debts and payment, before paper records replaced them:
Here /u/AncientHistory/ talks about a history of "computing" where much of it would have been people performing calculations, and makes a passing reference to Ming dynasty bureaucracy using an abacus: