basically to work out the offset of a wheel you split it in half (width wise) and then measure from the halfway line to the back face of the wheel that would meet the wheel hub. This number will be referred to as ET and the cupra's recommended ET for a 7x17 is ET35 to ET38. As a rule of thumb you can normally move +/-5 either way so if you car was recommended to run a ET38 you could fit a wheels that ranged from ET33 to ET43.
What confuses people is that as wheel diameter increases the width will generally also increase which will obviously increase the distance from the centre of the wheel to the back face of the hub.