@ Tigger ; find out for example the ET of the Cup Turinis. Compare this to your 172 FF wheels and the diffrence will tell you which spacer is neccescary to get the 172 Cup wide track.
Mind you ; you can'nt just throw on some spacers to your liking + go 2cm wider on each side. (well you could but..) The design of the car has taken account with the ET of the standard wheels, or the other way round.
Going too wide will put more stress on the bearings/hubs/etc.. and it will not handle better . It's possible but do'nt expect a nice/reliable drive..
The 182 for example has the bigger hubs, and to make up the difference the wheels have a ET to bring them back in some more, or so i was told by my Reno specialist..
I'm no expert but i have always tried to get the exact ET of the car design when getting new aftermarket wheels