Thinking about what it takes to change a chain length is easy. Look at the distances. As little as 3mm can force you to add or reduce a link. And one link usually means two, Ha. The chain length calculators just get you into the ball park. Just get it right and be satisfied, as I am sure you have done.

Of further note, all my bikes are Franken Bikes. I no longer get puzzled of the fact that even though they are set up similarly, chain length has been determined by trail and error...