Is an inch and 1/8
 

the same as 28.6? doesn't convert well 28.57500

Close enough. The labeled tire size frequently doesn't exactly match the measured dimension anyway.

Not a tire. Sorry should have been more specific FD clamp size. I don't want to pinch the seat tube.

Rounding error. 28.575mm is within .025mm of what you want or about .001". I doubt that the tubing itself, or the derailleur clamp inside diameter, is held to that accuracy and a coat of paint on the frame tube could be thicker than that.

Just a consequence of cycling's crazy mixed imperial/metric "standards". Take 1 1/4" (31.75mm) - you'll see in metric equivalent as either 31.7mm (uncommon), 31.8mm, or even as 32mm.