My last porter was 1.048, 81% pale malt, 8% dark crystal, 6% brown malt, 5% carafa special I, 0.66g/L First Gold at 10 min and enough First Gold at 60 min to give a total of 25 IBU. I fermented it with Wyeast 1469 West Yorkshire.
So with what you have I'd probably go with 85% pale, 10% caramalt and 5% chocolate for a simple version, drop the caramalt to 5% if you want it a bit drier, but I don't find crystal malts overly sweet as long as the yeast does a good job.