So here's an update:

I saw the nephrologist yesterday. He took one look at me and told me not to pay any attention to the GFR numbers. They apply to people of normal size and normal height.

Since I have one kidney and the contrasting chemical in the CT scans is kidney toxic, he's ordered a "doppler effect" ultrasound to see the actual blood flow in to my kidney. He's also ordered a 24hr urine collection and when I deposit that at Quest to check urine total protein/creatinine ratio, they're going to also draw blood and check creatinine clearance protein total, uric acid and an ANA panel comprehensive.