You're missing that the stars aren't all in a straight line. Two stars might be 200 LY apart but only 1 LY different distances from where you are now.The game shows you how far away you are from every system. Find the system you want, pick another nearby star and subtract. The difference is the distance between those stars. I have to be missing something here what is it? Do you need the decimals?
FWIW, the orange hats are the best way of telling whether you can reach a particular system or not. But they aren't much use once you get to neutron star boosting levels of jump range (200+ LY) as you struggle to be able to both select one system and see the other! Works well for the outer rims though where you're only interested in jumponium boosts so currently about 125 LY max.
If you are trying to use neutron boosts to get as far above or below the galactic plane as possible then estimating the co-ordinates and using Pythagoras is the only realistic option. You can estimate the co-ords to +/- 0.1 LY accuracy so can get pretty accurate distances.