Hi all once again great topic!
Personally speaking what is wrong with forgetting about the king post , just frame in a regular timber trusses every 12 feet, drop down a support rod from the peak of the truss to the centre of the span to support and keep the bottom chord straight, and then using a plate centre span on top of the trusses lay on your rafters overtop of the trusses, it will be still nice and true many years hence.
NH
