I really didnt want to chip in because reading "X kms to a tank" really shits me to tears, and usually results in an estimate of 20+ litres of fuel per 100km...Anyhow, either do the instructions that cowboy1600 lists and then bring us the numbers, or don't bother at all. No other estimates are going to be of use.
I would however add a few things. Once the pump clicks out the first time I usually wait 10 seconds then add more slowly until the second click just in case it cut out early. Ive had it happen.
Secondly, Ive had better results not avoiding boost. I have a auto stagea, which is also heavier than skylines. I had consistently worse fuel economy by disabling 4th/overdrive and avoiding boost. Try using less revs and more boost, thats the whole point of a turbocharger. Why buy a turbo car and not use it? YMMV.