If you (generic you) have trouble with stats, I find it's always helpful to turn them into frequencies. The 17 per 1000 is an average, over all age groups and averages over all women - the ones who have abortions, the ones who don't, all racial groups, all social groups.
Ignoring (to make the maths easier) variations in abortion rates with age (because in calculating the average that doesn't matter), we can follow a group of 1000 women from age 15 to age 44, i.e. 30 years.
Age 15 - 17 abortions.
Age 16 - 17 abortions
...
Age 44 - 17 abortions
Total number of abortions in this group of 1000 women, tracked longitudinally from age 15 to age 44
17* 44 = 510
Thus 510 abortions in a group of 1000 women.
Some women will have had one, some two, some multiple numbers. The upper and lower limits on the number of women having abortions (not the number of abortions in total) remain 1.7% and 51%, with a plausible number being somewhere between 1 in 4 and 1 in 3.
Yes, it's a back-of-the-envelope calculation. But it's a pretty good estimate. And tallies with the Guttmacher Institute's stats, and official NHS guidelines.