Track width is measured from wheel centerline to wheel centerline.
I would calculate the new and old wheel front spacing, subtract the old front spacing from the new front spacing, and then add two times the delta front spacing to the orig track width (TW):
New TW = Old TW + 2*Delta Front Spacing
= Old TW + 2*[(new wheel width-old wheel width)/2 -
(new offset - old offset)/25.4]
This assumes TW is in inches, wheel width is in inches, and offset is in mm (offset is usually listed in mm).