I'm not sure what he means by that - if you only have one kidney, seems to me it would only work at 50%, right?
My doctor has always measured my kidney function by creatinine - it was a bit higher than normal after donation but is now at .9, which is normal. "Function" as a percentage is not so much the issue, as I understand it, but how well it works to do its job.
Sounds to me like you need more information on what he means. I do know that the remaining kidney grows and eventually compensates somewhat for the lost kidney but you are never back to 100%. Is that what he means?