Good luck with Scapcento! I'm sure you can go as low as 4.5l/100km avg (52.27 mpg US) without big effort. And if you don't want to invest big bucks maybe you should go for vacuum gauge? That would be the cheapest FE gauge, and it's good enough to give you proper feedback. MPGUino is not expensive too I can help you to build one if you wish. Right now I'm building new one for Berta and that will be probably first MPGuino in diesel car (correct me if I'm wrong)
Anyway it's good to know that ecomodding becomes more popular in Poland.