The hammer spring adjuster (power wheel) shown in your picture is the newer version, I believe, than the one described in your user manual.
My DremLite has the older version which has increments of 1 to 5 + MAX, on one half, and A to E + MIN on the other half. This is the version you're referring to as described in your manual. And yes, I know that's more than seven, but I believe the A to E side are supposed to be mid points to the 1 to 5. So why they say seven I don't know.
My Crown has the version that you are showing. It provides continuous progression of hammer spring preload as you go from 1 to 21. I don't know that 13 is the exact middle point in that progression. Lots of things go into what your actual muzzle velocity, at each setting, will be with various projectiles. You need a chronograph and lots of testing.