What you said is actually incorrect, the more lower the number - the more out you go.. Stock is +50, so if you are +40 you're 10mm more out plus the .5" more than stock (8 inches) which translate to about another 12.7 mm so in total you're pushed out toward the fender 22.7 (a notch under half an inch).

The numbers are confusing less is more.. Usually sitting Flush you will probably want +30 to +35 on 8.5 inch wide.. if you are 8 inch wide probably +25 to +30..

I know, i'm actually running this exact setup, I have 18 x 8.5 +40 and its very close to flush in the front but rear needs 9.5 wide w/ +40, and that is how i like to roll and for the record its not really that sunken in because the wider rims 8.5, if it was 8 inches or less wide then yes Sunken for sure..