New construction home - estimate property tax?

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Hi All,

I just submitted an offer for a lot. It is going to me a custom home and will be my primary residence, but I may pull out equity once completed and use it for future investments. I'm trying to decide how much I can afford for the construction, but am stuck a bit on how to estimate the property taxes. I understand the county can use either cost basis or nearby comps, but that is going to be a huge range for me and will have a major impact on my annual tax bill. 

Cost for land ~$1M

Cost for construction - $1m-$1.5M

Appraised value after construction - $3M

Should I assume 1.25% based on a $2M house, $3M house or something else? This is in San Mateo County in California.