In other words, the firm is going to determine what each partner's contribution is to the after-overhead net profits of the firm. Partners need to be aware of the overhead, so they can feel it when it goes up or down. There are successful productivity allocation methods that do not deal with the allocation of overhead. The point is to try to measure the true profit contribution of each partner. Then one-third of net income can be allocated based on the relationships determined under this measurement.
Bonuses are part of such a system, and amounts vary widely.
But the tendency is to spread it around more evenly, like whipped cream on top of a cake. It aims to balance some of the inequities that come up in ownership and productivity weighting. Always look to the health of the firm's infrastructure. The managing partner should have power to allocate income. Without it, he or she can't do the job.