Aggressive offset and spacer calculation help needed
 

When I bought my Enkei RPF1 wheels I had no access to a fender rolling machine, neither did I know anyone who could do the job without messing up my fenders so I played safe and bought wheels which had the max possible width and offset without needing a fender roll.

Fronts: 18x9.5 +38
Rears: 18x10 +38

Now, I have access to a fender roller and I am planning to have a more aggressive looking setup. Looking at others' pics in the forum something like F: 18x10 +38 and R: 18x10 +18 would look good so I am planning to convert my setup to something similar by using spacers but I just need the experts to doublecheck my calculations.

According to my calculations to bring out 18x9.5 +38 like 18x10 +38, I'd need 5mm spacers at the front and to bring 18x10 +38 out like 18x10 +18, I'd need 20mm spacers at the back, is this correct?

As for tyre choice I'll go with 255/35/18 (diameter: 25.02) at the back but not sure about the fronts for flush fitment. My options are:

225/40/18 (25.08, perfect match diameter but too much stretch)
235/40/18 (25.40)
245/35/18 (24.75)

When tyres stretch, do their overall diameter decrease? If yes, then 235/40 seems to be the perfect match.

My final question is where can I get some ARP studs and quality lightweight (magnesium, titanium etc.) spacers suitable for rx7 centre bore? Do I need longer studs for 5mm spacers at the front?

Thanks in advance,

I guess I'm alone in this, so I ordered some 2nd hand tyres in following sizes:

235/40/18
245/35/18
255/35/18
285/30/18

Best method will be test fitting to see clearances and stance before ordering a brand new set.