In CRM → Reports → Pipeline stats this is calculated automatically:
Conversion stage A → B = (number of leads that moved from A to B) / (number of leads that ever passed through stage A) × 100.
The calculation runs 1x per day (at night) and weighs both active and won/lost leads. Low conversion between specific stages? Often that is your biggest optimization opportunity — e.g. between "Quote sent" and "Signed" 70% leaks = your quote or follow-up flow has a gap.