Is your sheathing vertical or horizontal? If it's vertical just put a board over the post and nail the plate, post and sill together, temporarily or permanently.
If your using horizontal sheathing then put one or two boards across the bottom to hold the post down to the sill until you've finished off the rafters, roof deck, and roofing. The added weight of all these materials will hold the plate, post and crown down.
In either case do this before you release your come-a-long, and you'll be alset.
It's only 3/16ths of an inch!
Jim