By all means check with your airline. If you end up convinced that they won't give you 21 miles leeway, consider this logic:

You will probably fly somewhere in the vicinity of 50,000 miles again next year. (Substitute some other number if appropriate.)

The bottom-tier bonus on that is 12,500 miles.

The mid-tier bonus on that is 50,000 miles.

The net gain is 37,500 miles.

If you take another flight to qualify, you will also probably earn a 1000-mile minimum for the mileage run and one mile per $ of fare for charging the ticket to your affinity credit card. Total, somewhere between 38,500 and 39,000 miles.

At 2¢ per mile, they're worth about $775.

No way is your mileage run ticket going to cost you even close to that amount.

Go for it!