Interesting, a coil shock on a Prophet. I had considered this for a while as I was having issues setting up my DHX Air. After discussing the matter with a few friends, I discovered that the combination of something as linear as the likes of a coil shock would have adverse results on a bike with a very falling rate of its own. My issue has been that I tend to blow through my travel far too rapidly; I wasn't bottoming out, too often, sag was fine...the suspension just wanted to react too much to every little hit. Ultimately, I needed a shock that would ramp up quicker.
While I have no advise for you in terms of spring rates, I do have some advise regarding your rear shock choice. I think a DHX Air would be more beneficial than the DHX coil...A DHX Air with low volume air can will be simple to setup and provide great results! If you choose to go with coil, I'd suggest solicit the help of PUSH Industries to get that thing dialed to your Prophet; otherwise, you will be very disappointed.
I am 175lb with a DHX5 coil on my MX. I am using a 500lb spring. So far it isn't so great. The small bump compliance is ok the the overall feel just isn't smooth. I might try a softer spring. So far I would say the Pearl is better at big hit absorbtion and the DHX is better at small bumps. I would have liked to try a DHX air but I couldn't get one after market with a small can like what comes on the MX1. There really aren't any good reviews on the DHX air however.
I've switched to a Roco TST R coil on my MX - much better than the stock Pearl. I've ramped up the bottom-out resistance and it tracks well, is supple over small hits and takes the big stuff in it's stride