Wheel spacer help

[tb8125]

Hey guys I am trying to fit some wheels on my Grand National And need some help ordering wheel spacers.. My question what is the spacer compensating for offset or backspacing.. Or should I be just trying to keep the wheels off of the frame and other suspension/brakes components. I had 1" notched out the rear frame to fit 20X10 0 offset 5.5" of backspace (no rub in the back)and the front is now on 18X8 0 offset a 4.5 backspace (with no rub lock to lock) What information do I need to calculate what size spacer I need to get the new wheels to fit ...I was thinking for the rear .8 or 1" spacers and for the front 1 to 1.25" spacer
Stock
15X7-3mm offset 4" backspace front and back

NewWheels
19X8.5 20mm offset 5.5" backspace up front
20X10 20mm offset 6.3" backspace in the rear