The System Hit Ratio formula is in the V3 and later Systems Administration Manual. This is much more meaningful than the Getpage/RIO value shown by most online monitors.
The HP retrieval ratio provide a quick snapshot of the benefit obtained from using a HP. When this ratio is low, less than 20%, the benefit of the HP should be evaluated. If/when this drops to single digits, the HP is just a wasted resource. If the pools are properly tuned first, and random object segregated form sequential, and highly updated from the rest of the world, HPs can provide great benefits.
The number of pages per write seems to average five or less in most installations. Lowering the VWTH to one or two percent will trigger the asynchronous write process faster, and avoid bursts of hundreds of writes when DB2 takes a checkpoint. Additionally, for large pool with a low Page/Write average, setting VWTH to ZERO will trigger the writes when 32 pages are on the write queue, and is the recommended value for large pools. The DWTH should also be lowered from its default value of 50%, usually into the 20% range or less.