I would tell you how I came to my offsets and widths but frankly, I wouldn't recommend my method.
Here are some thoughts about it after my mistakes:
I seem to remember that when I mentioned to Richard Winter (at Banks) that he gave me the impression that I didn't need tires that big (195/50-15 & 205/50-15). I think that's true however, I think the wider tires look better (especially on the rear).
Wider wheels (within the limits of the tire size) are better but narrower wheels (again, within the limits of the tire size) probably don't hurt much. The implications of this are that according to the chart below, I COULD have gotten away with my 205s on a 5 1/2" wide wheel (stock width for Brand Lotus wheel).
In case EuropaTC doesn't chime in, I'm pretty sure he has MG-TF wheels which from what I can tell are either 6Jx15 ET28 or 7Jx15 ET28 which means 6" or 7" wide with an offset of 28mm. My understanding is that he has no clearance issues but I don't want to speak for him.
Here are some sites that might help:
https://www.youtube.com/watch?v=pIqR8EQLISc (you're going to need an actual car for this)
http://www.etmags.com/pdf_download_documents/tire_chart_2010.pdf