My previous house had this issue.
Yes, there was functional obsolescence, when we lived there, and when we sold it. When we bought, there was a bath tub in second bath, under the stairs off living room, you couldn't even put your legs straight. I had tub removed and just had pedestal sink and terlit.
I found some reaction while the house was listed - mind you, this was a traditional styled house on a street with traditional styled houses. Many of the lookers asked the same question, where could they put the second full bath. House had an enclosed porch that was one possibility.
Anyways, when it was time to give kids a bath, and hubby was sleeping just before work, it was trouble. When kids wanted to shower in morning before school and hubby was sleeping, it was trouble.
How can you determine the amount? Good question. You probably won't find another house with the same issue, but why not look for a house with some other obsolescence.