I’m not sure what you’re looking for. I am a long way from learning or using much math, but...
The adjustments to labor are applied separately.
First, the port’s reduction:
2000 x .6 = 1200
Then, your farm’s level reduction:
1200 x .8 = 960
I assume somewhere in there is a rounding up to 961.
I really hope I don’t now have to copy and paste this on my Facebook page and tag my friends.