The devil's in the details. It's hard to come up with measures and targets that are both fair and easily communicable.
If you go with number of people, well, that's the wrong measure (and hard to enforce // easy to fake). Number of bedrooms is also incorrect, as is square footage. Number of rentable units is also weird, since it encourages studio apartments and the like at the expense of splitting bigger units and sharing areas.
Another option is using a baseline of "however many people currently live there", but that favors parking lot owners and the like. "Average density of the local region" is unfair in that it suddenly imposes a new cost on the density hold-outs.
You can calculate a target based on something like distance-to-transit and local zones, but that gets complicated and political in a hurry. Land-value would be a good choice, since the price conveys really useful information - but that is likely illegal due to proposition 13.
My next-best idea is collective punishment: charge zoning districts based on the difference between how their residential and commercial/industrial construction. Mountain View is much less likely to vote to let Google expand their campus while blocking commensurate housing construction if it means they have to start subsidizing the places those workers start commuting from.