Can I use this estimate land value divided by 48 as the value of the land?
I don't see why not. Just be aware that whatever value you give to the land is "a part of" the original price you paid for the unit; not a cost separate from it.
Typically, land values will be around 30% of the total cost of a property. But then "typical" is a house and lot. You have a unit within a larger structure. With 48 units I see no issues with the land value percentage being extremely low.
