go measure four different tyres, all 225 sized.

they're not all 225mm in section width.

Tyre sizes vary between manufacturer. section width doesn't always mean the tyre is that size. It varies.

You can apply basic maths to the situation but it does vary slightly. You can crunch the numbers to give you a guide, but based on which tyre you choose, it may differ slightly.

Plus the differential between tyre section with and rim width will cause different tyre mounting points and different sidewall deflections, which will change your mounted tyre section width.

Something to think about ;)

This is correct. You can tell that two tires have different widths even if theyre both 215 just from looking at them.

thanks for the help guys, decided to go for it and order these wheels in chromium black so it comes in before my car comes in jan :)

just gotta decide on a set of tyres for daily driving with better grip than the oem ones and lasts a decent amount of k's without raping my wallet

Oh I realise all of that already ;) That's what it is basic maths, not exact science ;)
I realise that a 225 isn't exactly 225mm, and some are wider than others, but it gives a basic idea of how the tyre will fit on a given rim ;)
Nothing new for me to think about there, but thanks anyway :)