A sherman 75 with no veterancy fires at something for the first time at 999 meters.
It has 5 accuracy, which is modified -1 due to no veterancy.
a 2d6 is rolled, if it at or under 4 (a 1 1, 1 2, 2 1, 2 2, 3 1, 1 3) it hits. As there are 6 possible successful rolls out of 36 total combinations the chance to hit is 6/36 or approximately 16%.
Each veterancy level accuracy is increased by 1, so a single veterancy unit behaves as you would expect its accuracy value to intuitively.
Each shot fired without moving at the same target increases the accuracy by 1, up to +2 I think, so the third shot and onwards is the maximum accuracy.