Capacity is not how many technicians you employ. It is how many technician-hours exist next Tuesday.
Why it matters
A planner can only work with the hours it knows about. If someone finishes at three on Fridays and the calendar says five, every Friday will be planned an hour and a half too long, forever. If training days are not in the calendar, the work scheduled for those days has to be rescued by hand.
This is the least glamorous data in the whole operation and one of the highest-leverage, because it is wrong in a repeating, predictable way.
How you notice it slipping
Routes are planned for people who are not there. Part-time patterns are handled by remembering rather than recording. Absence is entered after the schedule was built, so the schedule never reflects it.
What to do
Treat the calendar as the source of truth and get the exceptions into it before the planning window opens, not after. Then compare committed work against available hours per week. When that comparison is honest, most “we need another technician” conversations resolve themselves in one direction or the other within a month.