I am a lot more familiar with conventional keelboat rigging so I am sure others can add to or refute what I have to say, but it is my understanding that on a rotating mast the shrouds are not functioning to shape the mast but only to hold it up. The diamond stays along with downhaul and mainsheet will control mast shape/sail shape. So on the catamaran with rotating mast your shrouds need to be tight enough to keep the mast from falling off/flopping around and loose enough so that the mast can still rotate. So really shroud adjustment I would suppose is just a matter of how long you have your forestay set, but forestay tension is still controlled by the mainsheet. It sounds like you have yours about right. The only reasons I know to adjust them is that it can be advantageous in light air to have slightly looser shrouds so that downwind your mast can rake further forward or in a capsize if you extend a shroud on the side out of the water it can make recovery easier.