The phrase "a fraction at" is correct and usable in written English.
It can be used in contexts where you are referring to a specific portion or part of something, often in mathematical or statistical discussions.
Example: "The report indicated that a fraction at 0.25 of the participants preferred option A over option B."
Alternatives: "a portion of" or "a segment of".
He came second by a fraction at the 2006 world championships and last year he won the 1,000m and was second in the 500m.
Open from 9 a.m. to 8 p.m. daily, it is 14 feet high and can accommodate 400 people -- though only a fraction at a time are allowed at the edge, which directly oversees ground zero.
Then on June 4, Etecsa, the state telecom company, opened 118 Internet salons around the island, expanding public Web access — by a fraction, at least — in what is regarded as the least wired country in the Western Hemisphere.
Since our story about how donors could save on taxes by giving away artwork a fraction at a time, such gifts have become far more common, museums tell us.
Glancing to his left he was aware of Blake closing in a fraction at 150m, but held his form to cross the line with a finger to his lips.
I kept moving a fraction at a time.
Only a small percentage will succeed, but only a fraction succeed at the highest level in any profession.
The washed eluant was collected using a fraction collector at 1 mL per tube and assayed for enzyme activity.
Treatment at 6 days before RT led to a survival fraction at 2 Gy and a D0 value of 0.46 ± 0.05 and 2.2 Gy, respectively.
AB grows to a peak point of 0.3162 (as a population fraction) at (t=226,) and quickly diminishes.
Further analysis of the calculated difference spectra of HmHR and HwHR indicated that the decrease of chloride concentration made both HRs show a decreased fraction at approximately 550 nm and an increased fraction at approximately 650 nm.
