121.617b says the takeoff alternate has to meet the alternate minimuns in the Ops Specs. Paragraph C55 is the create your own minimuns paragraph based on the available approaches. Pretty much for one approach add 400 and 1 to the mins and for 2 approaches, 200 and a half to the higher minimuns. The approaches have to be to different runways unless you're ETOPS, then they have to be to separate runways. If you're good for CAT III and the airport has dual CAT III runways you can get your alternate minimuns down to 200/1800RVR. If it was down to that, I'd see about having a second alternate added to the release.