The percentages mentioned on the "Other Things to Do with Deltas" page are: the gap measurement (Step 2) divided by the distance from the nose from Step 1, expressed as a percentage (Step 2/Step1 x 100).
For example, if the measurement from Step 1 is 28.75 inches, and the gap in Step 2 is 4 inches, the percentage is (4/28.75)x100=13.9%.