single is 30mls in 30 seconds, double is 60mls in 30 seconds (plus or minus a few seconds and a few mls)
so you can pour your double into two cups and have two seperate drinks with a single shot each.
Adjust your grind, ideally keeping the tamp the same until you get close enough to the figures at the beginning of the post.
Too much volume in too short a time then the grind is too coarse.
Not enough volume in 30 seconds and the grind is too fine.
If your numbers dont match the ones here and the drinks taste good then you must be doing it right ;)