I have posted a repy to this sort of query before but here goes.
The offset of a wheel is the dimension from the geometric centre of the rim width, to the wheel mounting flange.
It is usually expressed in mm, + or - depending on whether the rim sits towards the centre of the car or towards the outside of the car.
That dimension is for a 1600 around the +30mm mark.
Back spacing is the dimension from the wheel flange face to the inside bead on the rim.
As the rim width varies for a constant offset, the back spacing will also vary.
So, getting back to your question/problem, the maximum back spacing any strut can handle is the distance between the wheel mounting flange on the hub, to the strut tube at a point equal to the rim diameter, minus a fiddle factor which is the tyre section width.
A wider section tyre will overhang the rim and may rub on the strut tower.
For example, if the above dimension was 105mm, a rim with 100mm backspacing will clear the strut but when fitted with a tyre, any tyre, it will probably rub.
Flared guards will allow you to reduce the offset from +, to zero, or in extreme cases, start going -.
If you are buying from a dealer, it his his responsibility to ensure the rim/tyre combo fit and are legal.
If you are buying privately, there is an old Ethopian saying, "Caveat emptor", which means "Buyer beware"