Technical Wheel offset & back space?

My dyslexia screws me up with these two terms. Question, if I have 0 offset w 4” back space now & want to move the wheel further under the wheel well by 2” would I go to a -2” offset & 6” backspace?

 

That's the way I understand it.

 

My best advise has always been to just not use the term "offset", especially when ordering any kind of custom wheel, you might "think" you understand it but if not it could be an expensive misunderstanding.
Pretty hard to mess up by using "frontspace" and "backspace", pretty standard terminology nowadays.
The other mistake people make in calling out wheel width is using the overal outer edge dimensions in measuring wheels, it's the dimension between the bead seat flanges that dictate wheel size.

 

Actually wheel width is measured using the inner dimension of the bead flanges, but backspace is measured from the outside of the flange.

Offset, it doesn't matter which you use, as long as you use the same surface on both sides of the wheel

 

Offset is based off of the center plane of the wheel, which is located halfway between the two bead surfaces. This is assuredly the most accurate method as it isn't dependent upon the height of the lip on the backside of the rim, which can vary from rim to rim for a given width. OEMs and wheel manufacturers specify (or can specify) their wheels using offset.

A negative offset will have the hub surface located in the inboard half of the wheel (towards the car's centerline).
A positive offset will have the hub surface located in the outboard half of the wheel.

So, in your case, you're increase in backspacing by 2" also increase the offset by 2".

 

On a full fendered car where space is at a minimum the tires' sidewall bulge needs to be a consideration too.