how do you do it with out a chrono?

parker buck hunter xp
392gr arrow
30" draw length
55lb draw weight

You can do it if you know the IBO of your bow. Let's say your Parker Buckhunter has an IBO of 286. That means, with a 30" draw length and 70 lb. draw weight, a 350 grain arrow would be going approximately 286 fps. Going down to 55 lb. is going to cost you about 30 fps. And the 392 grain arrow is going to cost you another 14 fps. So your speed is going to be in the neighborhood of 242 fps.

so 1lb of draw weight is about 2fps?
and 3gr of arrow is about 1fps?

how about KE

I would have guessed 230 - 235, but that with with a peep and a loop.