A typical formula: drafts sent unchanged times the minutes a reply would take to write, plus a smaller credit for drafts that were edited. Tools choose their own minutes per reply, so compare the assumption before comparing the number.
Why it matters
Time is the reason to use a drafting tool at all, so the estimate is useful as a trend. As a precise figure it is weak: a two-line reply and a careful complaint answer do not take the same time, and reading and checking a draft is work too. A more honest view combines the estimate with the sent-unchanged rate, which shows how often checking took only a glance.
You can also measure it directly for a week: note how long the inbox takes with and without drafts for the same kind of email.
An example
A tool reports several hours saved last month, based on four minutes per reply. The team checks: for order questions that is about right, for complaints it is too generous, because they still rewrite those.