They say an ounce of luck is worth a ton of judgement.
By your method you clearly can't measure future performance.
Plenty of other factors besides the two you mention,tempo,barriers,weight...etc.
Don't know how many times I've heard the post mortem...needed luck...couldn't get out in time,an ounce of luck it wins,unlucky got caught behind a tiring runner,got bumped at the 500...and so on.
So luck is most definitely a big factor,when there are only seconds between 1st and...last.