You should get around 40mpg on non town/motorway driving, bit more or less depending on driving style, 45mpg is easily possible should you wish. Thats around about 350-400 miles on a tank when you fill it from the red on the gauge (approx £60 worth).
You mentioned using a website to calculate you MPG, you can do it easily with the calculator on your phone....
1. Fill the tank till the delivery pump clicks off.
2. Zero your trip counter after filling up (the one on the clocks is best as its never off like the MFA display)
3. When you next fill up, the trip counter tells you how far you went on the last tank of fuel
4. repeat step 1, fill the tank till the pump clicks off.
5. take the number of litres bought from the pump display (LITRES) divide it by 4.54 to get it in GALLONS:
LITRES / 4.54 = GALLONS
6. divide the distance on the trip (MILES), by the GALLONS used to get MPG:
MILES / GALLONS = MPG
7. Do this every time you fill up to get a feel for what the car is doing and to see what difference replacing parts and setting up the engine has made.
good luck.