How much to take off of 6/4 lumber, when planing?

I ordered some 6/4 lumber from a local Amish sawmill.
It's now been cut, and delivered to my dryer.
Should be coming out of his kiln any day now.
He also offers surfacing of dried stock.

I bit off more than I can chew, so I'll be selling some of this. I simply ordered too much.
How much should I ask him to plane off, such that I can truly sell it as 6/4 ?
He told me he'll plane it down to whatever I want....

(.-.)

How much to take off of 6/4 lumber, when planing?