this mainly is for cars with manual transmissions, when you are going down a big hill rather than slip it into 'angel gear' and ride the brakes you leave it in 5th or whatever gear and use the wheels to turn the engine over. there is no need for it to inject fuel as it doesn't need to make power. if you are getting that tight about fuel life must be getting rather dull.
the half filling this just seems false economy to me, you then have to make many more trips to the servo to get fuel, where as if you just did one big fill you wouldn't need to travel all the extra kms to go get fuel. unless you have long range fuel tanks causing you to carry over 100lts of fuel the extra weight is just not worth worrying about.
if you are getting that picky with things, don't use the heater/air con. the fan causes the alternator to load up using more fuel. don't use the radio, that also will use fuel. if driving at night use battery torches, head lights use a heap of fuel.
personally, i drive a big car (5.3mt long) which has a high performance straight 6 4.0lt engine, the climate control is always used, meaning the air conditioner is always used, i always drive with my headlights on (safety) and i always fill the tank to the top - 70lts of fuel. my car has a full sized spare wheel, full of leather and woodgrain and sound deadening. around town it uses a fair amount of fuel, i am lucky to see 250kms to 60lts of fuel, however out on the highway it really shines to be the long distance cruiser it is, my fuel economy AVERAGES out to be around 7-8lts per 100kms. my 1.6lt 4cyl mazda uses about the same amount of fuel, little cars are good around town but on the open road the little engines are working so much harder causing them to use so much fuel. little cars are not always the best option.
if you are serious about saving fuel, put about 35-37 PSI in the tyres, don't use climate control if you have it, the air conditioner will kick on and off causing you to use more fuel, don't use your head lights when you don't need to ( however the extra in fuel is cheaper than a crash) and also use a high octane fuel, i get much better economy by using 98 octane. Have you thought of converting to LPG? however the weight and size of the gas tank is a disadvantage.