Adjustable feedback schedulers control the relative performance obtained from a computer system by different classes of users. This work examines the control of relative throughputs (or related measures) by feedback of departure counts. Counts from key resources in the system are used to dynamically adjust the priorities of the different classes at certain queues in an attempt to achieve preset values of the ratios of the class throughputs. An iterative mean-value algorithm is given which approximates the throughputs actually achieved by this scheduler within a few percent of error. Limits of controllability are observed beyond which the achieved throughput ratios cannot follow the preset values. A characterization is given for this `controllable performance set'. It is also shown how, using this set, the scheduler can be tuned to have approximately optimal set points for the ratios.