This was on your D800? My 24-70 needs -12 on my D800, but no AF fine tune on my other cameras. I attribute that to the D800, as several other lenses (but not all) also need significant negative AF fine tune values. At some point I will probably have my D800 adjusted. The backfocus of the D800 with some lenses may be fairly wide spread, especially with early bodies. I have seen it on all 4 D800 bodies I tried (the previous 3 bodies also had the left AF issue so I had to return them).
So before you even consider doing anything with your 24-70, try it on another camera.