OK I'm really a bit flabbergasted. I looked through the entire Oolite source code to determine the function that calculates distances between two systems. To be sure I get it right, I took the example of Lave <-> Leesti, which is 3.6 LY - according to the game.
However if you apply euclidean formulas and take into account that on the x-axis every step is 0.4 LY, and on the y-axis you have 0.2 LY, you get out a distance of 3.9 LY.
And here's the kicker... apparently even Oolite itself thinks it's 3.9 LY. I found the code that does it:
https://github.com/OoliteProject/oolite ... dom.h#L166
I found the reference to the function first in this line, which is - I think - responsible to draw the map in the F6 screen:
https://github.com/OoliteProject/oolite ... en.m#L1899
I wrote a small tool in PHP, adapted the formula as found in above GitHub gem... and it's exactly 3.9597979746447 LY according to PHP, using that very same algorithm. I used my own formula before, and I got the same result.
So now comes the obvious question...
How does Oolite calculate the distance between two systems?