It was trickier with VB6 because you only had the 90% of remaining fuel order, meaning that if you miscalculated, your tanker would get stranded between planets. With the absolute amounts that C# gives us in regards to tankers (the minimum fuel amount is liters, not percentage), you can calculate the amount of fuel the tanker needs in the worst-case scenario between Earth and Jupiter, put that number in as the minimum amount in the class design MISC window and then just leave the tanker doing it. I wouldn't give it more than 500 000 liters of capacity since your harvesters are small. Let it run for a few months and see whether your harvesters are empty or full - then either add harvesters or add another tanker.