ET question

i am looking to replace the rims on my plow truck and i am a little confused
(the stockers are long gone so i only got internet date to go on)


what i found is that stock wheels were 18X8 with an ET of 40

the wheels i want to get are 18X9 with a ET of -9

if the wheels were the same width, i would say this would push them out 49 mm, but with the difference in width, would that only be about 36 mm?

(i will try to see what exactly i have on there now, to complete the picture)