Track problems solved per week.
Practice volume is measured as a rate, not a running total, because the total only ever goes up and tells you nothing about whether you are keeping pace. Dividing problems by days tracked and scaling to a week gives a figure comparable against a target, and converting it into hours shows the time commitment the rate implies. The target comparison is signed, so a negative figure is an honest shortfall rather than a hidden one.
Rate
Problems per week = problems solved / days tracked x 7
Time and target
Practice hours = problems per week x minutes each / 60; gap = rate - weekly target
Yes, if you want a sustainable rate. Dividing only by active days flatters the figure and produces a plan that collapses the first week something else comes up.
Only alongside difficulty. Fifty easy problems build speed; ten hard ones build capability. If the per-problem time is falling while the count rises, check that the difficulty has not quietly dropped.