I just received 105' of Ukmanen 26 ga Ti grade 1. Using a precision digital caliper I am reading .42 mm which is a little higher than what SE says 26 ga should be .405 mm.
For a reference:
25 ga - .455
26 ga - .405
27ga - .361
So the Ukmanen 26 ga Ti is a little over 96% accurate being slightly on the thicker side. I would think this is good enough for government work, no?
This is from the Rio Grande site, but I'd bet it applies to most vendor wire:
"please note: We strive to measure quantities ordered as closely as possible; however, due to manufacturing tolerances at the mill, please allow for a ±10% variance when ordering."