Outcome

What the bench costs you

Every hour nobody is booked for is paid for anyway. Work out what unbooked capacity costs over a period, then see who has room next week.

What it comes to

In words: Multiply the people by their weekly hours and the weeks in the period, take the unbooked share of that, then value the unbooked hours at what an hour costs you.
In symbols: Bench cost = P × H × W × U
Unbooked hours = P × H × W × U × C
Full-time people idle = P × U
With your numbers: 20 × 40 h × 4 × 15% = 480 h
480 h × 45 € = 21 600 €

The loaded cost per hour is salary plus employer costs, divided by the hours a person is contracted for.
Over-booking someone does not offset the bench: a person booked at 120 % does not make another person’s free hours cheaper, so the result is never negative.
Approved time off is not capacity. Take it out of the hours before you estimate the unbooked share.

1Capacity is each person’s own contracted week and work schedule, with approved time off taken out, so nobody on holiday or part-time looks free when they are not.Read the guide
2The bench lists who still has room in the period, most free first. Drag someone onto a role to sketch a draft booking; drafts never count as coverage until an approver confirms them.Read the guide
3Actuals compares each person’s planned hours with the time they reported, week by week, with billable utilisation and where the hours went.Read the guide