Cementing Posts For a Fence

I am building a simtek fence. I live in Saskatchewan where we have frost 3' deep. I was advised by the company where I am buying the fence to go with 4 1/2 foot holes for my fence and cement the post in. My post will stick in the ground 2 1/2 feet.

I am thinking of drilling 5' deep and was wondering if I could fill the bottom 2 feet with gravel and have rebar running to bottom of the hole have it running up to about a foot from the ground and then pour the Instant Post cement in on top of the gravel to just below the ground?

Would this work or should I do Instant Post cement for the whole 5' ? Any feed back on the Instant Post cement?

They say just pour the powder in the hole and pour a little water over top and it will suck in enough moisutre to get a good cement job.

Any tips or recomendations are appreciated.