I'll do my best
Okay... I'll start from the start. When you ID a ship and hit the checkmark it enters the mast height into the range part of the TDC. This is the clear triangle with the red tick mark in it around the outside of the wheel.
You can also enter this manually. At any rate when you use the stadimeter the inner wheel of the dial turns and is supposed to do that math for you and give you the range.
So if you want to forgo ID'ing the ship and have an idea of what the range is just ignore the image and move the stadimeter until the wheel turns to the desired range.
Now to answer your question greekfire. If through observation or using the attack map you realize that the range is off you can tweak it by changing the input of the mast height a little this way or that. You do this by moving the clear triangle with the red tick on the outside of the wheel. If you move it clockwise it will increase the range input and vise-versa without having to take a new stadimeter reading, be sure to hit the send to PK button after moving the mast height.
Obviously this isn't an authentic way to do things, but this is war, what ever works.
Disclaimer- There are of course many ways to skin a cat, but this has been working for me when I want to make relativly minor range adjustments.
-Happy Hunting