I've read that if the IRS were to audit me, they would (s/b could) want my rationale for the ratio I chose. The same with any other ratio. Even though your tax bill doesn't break out land vs building even when they do, they may not reflect true valuations. if you want to have some defense pay a realtor to do an analysis. others pick numbers out of the air and hope they're never audited. others may offer other suggestions.
here's an article from a major accounting firm on your situation.
https://www.kbkg.com/tax-insight/how-to-allocate-land-vs-building-values-for-investment-property
