Client profitability is what the P&L hides. The agency average might be a healthy 25% net margin, but the underlying mix is usually one client at 50%, one at 5%, and one quietly negative. Computing the number per account changes which fights you pick.
For agencies, the calculation is straightforward and the discipline is hard. Take fees for the period, subtract directly-attributed costs (the team loaded hours on that account, freelancers, pass-throughs), and you have gross profit per client. Subtract an allocation of overhead and the picture sharpens further.
The reflex move on an unprofitable client is to fire them. The better first move is to fix the cause: raise rates at renewal, tighten scope, or move them to a less senior team. A client that becomes profitable is worth more than a slot on the bench.