I have 2 SFR that were purchased and placed into service this year. I have one more that was a personal residence that I placed into rental service at the beginning of this year. How do I determine what the depreciation basis for these properties is? Is it a 90/10 split? 85/15? Do I need to look it up on the county records? What are the rules? The first time I did this the tax advisor who was advising me simply said it's always a 90/10 split but I want to hear what other people think?

90/10 is just a rule of thumb.  It may make sense for where you live but if you have waterfront property in California, the land is probably worth more than the house. There it might be 50/50.

Your county assessor will have values for the land and for buildings. You can almost always ignore those values as they don’t mean anything. But you can use them to get a ratio of land to building values. 

I am not a lawyer or accountant and this is not legal or tax advice.

What I did: there is a vacant lot a few blocks away from my SFR. I did some digging and it used to have a house on it, but it burned down several years ago. I also found a recent-ish "for sale" ad for that lot, and I looked up the valuation at the county, which was in line with the sale price. So I took the value of that lot, scaled it down a bit according to the square footage (my lot is slightly smaller), and called that the land value of my lot.

I was comfortable doing it this way because I felt like the vacant lot was of similar quality to mine - same neighborhood, not on the big through street, close to the same size, and so on.

That makes sense. I bought the vacant lot next to my investment property for $250 dollars. So that could be close to the value theoretically. 

@Christopher J Lemmon

I would be wary of an accountant who says its "90/10". That will not stand up in the case of an audit from the IRS/State.

The IRS has prescribed many ways for you to determine depreciable basis for the building.

One of the most common ways is to look at the county assessor's website and see how they broke down the value of land/building.