Its really not difficult to grasp.
The higher the offset the further in the wheel will sit
The lower the offset the further out it will sit.
The RAYS are ET30/33 & with a 20/25 combo become 10 & 8. So therefore ET30 will require 20mm spacers & ET43 will require 35mm spacers.
A spacer reduces your ET by whatever size the spacer is.
Personally, I'd just buy some wheels that fit. Those wheels were not designed to go on a Z so get something that was
The lower the offset, the further OUT they sit. The higher the offset the further IN they sit.
An ET40 is higher than stock, so therefore will sit further IN than the stock wheels.
The ideal offset for a Z is between ET15 & ET 25
It used to belong to Gazhull on here & then another member who's name I can't remember - he attended a SW meet last year though.
Theres a topic on this car on here somewhere, but basically the guy doesn't know what hes talking about
I went from Cardiff to have my car done by Abbey (Twice because they finished stripping it for me aswell ) & I'll be back down there in the next month or 2 getting the Skyline mapped