Ian's method is the usual one and it may be the best.
Another procedure is to make a test strip for this specific print. First establish your best print without flashing. You would have to do this anyway, whatever method you were using.
Take a test-strip sized piece of paper and expose the part that needs correction to your best print time. Place a steel ruler, or something similar that won't move, along the strip and give it successive flashing exposures by whatever means you choose, at right angles to the ruler. You will need to keep count and it might be worth marking the intervals on the paper before you make the exposures.
Now develop as usual. You are looking for the first difference between the shielded paper under the ruler and the test-strip part. By using this method, you will have a choice of exposures. The advantage of this is that sometimes you might welcome a more-than-minimal effect, such as when there is a very pale sky and the horizon is too complicated for satisfactory burning. Conversely, you might have white horses on a dark sea, too many burn individually without risking haloes where the burnings overlapped.
As Ian says, you might have to adjust the main exposure slightly, too.