If you look up a random house on Zillow[0] and scroll to the bottom on the right in the "Neighborhood" section, it will tell you about how much it's gone up/down in the last year and their prediction for next year. I've looked at a lot of houses and the 10% is sort of my mental average of what I've seen around the Bay Area. That house for example has a 13% down predation, but each "Neighborhood" is different.

[0] https://www.zillow.com/homedetails/5441-Kaveny-Dr-San-Jose-C...