The Neptune wheels are 7.5 ET51 or ET52

You have to calculate the distance each rim edge is from the wheel (offset) face
Do that for the Neptune and the after market wheels
Then compare the new wheels to the Neptune measurements to determine the difference

8.0 ET45 will be the same distance to the shock and poke out by 12mm
8.5 ET45 will be 7mm closer to the shock and poke out by 19mm
(I think 8.0 ET45 is 12mm~13mm from the shock)