Something to keep in mind: FFE comes with 225/50-17 tires as standard, as opposed to the narrower and thinner 215/50-17 tires on gasoline models (SE with optional wheels, base Titanium).
Current stock 18" tires on the Focus Titanium are 235/40-18. Going with 225/40-18 would be a smaller diameter, let alone compared to what you have on your FFE.
I would use a tire size calculator and maintain a diameter as close to stock as possible. For the FFE, my hunch is 225/45-18. But keep in mind 18x8s and performance tires will most definitely affect your range and MPGe.