I second this information ... last week i just went in and had my tank filled to a 3/4 first, +/- 1 % from the gauge, then i pump another time to make sure that I have 3/4 after the traveling from SengKang to 2nd Link (woodlands jam from onemotoring)
the differences is only about another 12 dollars, which is about 5+litres
I usually will drive till reserve light up and most of my fillup is about 38 litres
so technically we have about 7 litres of reserve but please bear in mind that the design usually have about 2 litres of buffer and our fuel pump will have difficulty draining the remaining 2 litres, thus we will be only able to travel for about another 4-5 litres. 
but i seldom do that as I do not want to stop my car waiting for help ... 
Hi 553,
Question: Are you related to 883?
Just kidding.
Well, I don't think is worth to go JB to pump petrol then come back to SG.
Unless, you are going to Msia for a short trip, at least half to 1 2 days. Otherwise, it is not worth at all.
1) Jam when you go? Enjoy
2) Jam again when come back
3) 3/4 tank b4 you exit
4) Pump abt 1/2 tank before you back to sg (still worth?)
Msia inflation is much higher than SG, I believe for shopping, SG would still be offering the cheaper price for most of the products.
My 20cents.
Regards.