You don't want to do absorption rates based on zip code. You really should localize them more to neighborhoods. One neighborhood might have a 3 month inventory and another a 6 month one. They could be in the same zip code. You also need to make sure you are looking at comparable inventories. Inventories of 1300 sq foot starter homes is different than inventories of 2500 sq foot 2 story homes. I just ran inventories for new construction homes in Salem OR and it is 19 months, but resale homes are at 6 months. That makes a huge difference in how you price.
Get a good agent for help. My .00001 cent worth.