One thing to remember is that for small quantities the SG does not make a lot of difference. Our airline uses a standard SG for all short haul aircraft of 0.800. The difference for 8000kg from summer to winter SGs is only about 300kg. Obviously if you are putting 150000litres on a B747 it makes a difference, but not worth the effort on an aux tank on a business jet.
The refueller who fills up the tank will always give you the SG which will depend on the source of the fuel and where it is stored. If it comes out of a pipe line it will be well below ambient
temp.