I always use trial and error. The most important thing is to keep the checkpoint duration as small as possible.
If you have a big chunk of memory available in your system, bring up the BUFFERS parameter and fine-tune with LRU/CLEANERS/LRU_MIN_DIRTY/LRU_MAX_DIRTY.
Make sure that your checkpoint duration stays low.
Be aware that not only the BUFFERS parameter affects the total amount of memory allocated by the informix server.