With my larger ships (32t tanks) the script works perfectly however in my smaller ones (Courier, Cobra, etc), I too get the 75% warning on every jump. When I isolate the variables, they seem to give the correct values so I'm looking into the decimal values now.
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Also watch out for the "< 5" line. This is checking if you have less than 5 tons of fuel left. Some of my ships don't have 5t total.
I'm checking it out now too while flying my Anaconda back from Maia to my home-system and I found it only uses the main tank for telling the percentages. I have one extra 32t tank aboard so 64t in total. It announces the remaining fuel correctly (like 58t after my first jump) but for the percentages it only looks at the main tank (32t) so the 25% depleted starts at 24t, 50% at 16t and 75% at 8t remaining. I'm thinking about making a VA routine to fix this since I don't think the extra capacity is available in the scripts.