Yes, if it has only ever been a rental, then use was 100%.
When you initially set up the rental property, you would have separated out the land, since the land is not depreciable. You can use that as a starting point to calculate the land value now.
If you don't think there has been any change over the years, you can use the same percentage you initially did.
If the purchase price was $200,000 and the land value was $20,000, then allocate 10% of the sales price to land. If you think that that land has increased in value in relation to the house, you can increase the percentage.
