wheel size- 17x8 difference between 0 offset,4.5 backspacing VS 12 offset,4.89 backspacing.....IS THE 12 OFFSET WILL TUCK CLOSER TO TRUCK ONLY 1/2 INCH...... correctamundo?

Yes.

Actually, it will be around 0.47 inch closer. (1 inch = 25.4 mm)

A 17x8 wheel with a +12 mm offset should have close to 5'' of backspace.