I guess no, there is no such equation. First of all because when fetch instruction is applied by wavefront executing on compute unit, this wavefront goes to fetch unit, where it sits until fetch is done. At this time other wavefronts are doing calculations, or wait unit fetch unit become free, to execute next fetch instructions. So when some wavefronts are doing memory read or write other can do computations, and in the best case both counters can have 100% value, and ALUFetchRatio counter will equal to 1. Another important counters is FetchUnitStalled and WriteUnitStalled, try to keep them about 0 value. If it's too big, then many of wavefront are waiting for fetch unit to do memory read/write. To improve performance first of all, try to use sequential memory access pattern, then try to use local memory, if your algorithm reuse data several timers within workgroup.