In Gordon R. Dickson's Dragon Knight series, the plot usually revolves around the main hero Jim Eckert's limited magic account.
Initally, as class D magician/apprentice, he can't do much unless he draws on his Master's, AAA+ class magician's S. Carolinus account. During the course of next books, he gains higher rank and also some more magic in his account, which nevertheless can be too little for the dangerous and intricate tasks he is drawn into.
Jim is constantly in situations where he is not sure how much magic he has left and whether it will be enough to do the job. Is there any in-universe explanation somewhere, why Jim never asks Carolinus how to calculate the account value and how to estimate how much of the account would any performed magic use up?
