This Wheel and Tyre tool is excellent for checking what offset/wheel/tyre combo's work ... obviously input the original wheel and tyre specs in the left hand side and play with different combo's in the right hand side and check the differences.
http://www.rimsntires.com/rt_specs.jsp?i...xt1=&text2= Personally I wouldn't go anymore positive offset than the original 52.2 as there is almost no clearance between the inner edge of the rear tyres and the wheel arch shroud ... why would you want to tuck your wheels any further under the arches anyhow? ... you'd want to bring the wheels out more surely?
The best setup on the Fiesta ST is a 17" x 7" wheel, ET40 offset with 205/40R17 tyres ... looks great!
That combo will obviously bring the wheels out a further 12.5mm more than the stock wheels .. fills the arches much better.