you will find a recommended mast length printed on most sails. LUFF is the entire length of the sail incl. extension, then there will be a recommended mast length. now adjust your extension to the difference between the two and away you go.
IMCS will matter if you super worried about performance. It will affect the way the sail twists of and powers up. but if all you want is a stick to throw a sail on then it won't matter too much.
im thinking about buying a north natural 6.6 it takes a 460 mast imcs 24/26 has a luff of 475 and boom 196,i have a 430 mast imcs 21/25 and a boom198 max,just wanted to know if this sail would work on my gear
It will work as long as you have a 45cm ext. Just remember with your boom at max extension you will lose the rigidity and it will become more susceptible to damage.
I've been contemplating a bigger sail too, but am in two minds as to what i should upgrade first. Mast 430 - 460 or boom 145/195 - 180/230?? If only money wan't an option 
I'm with ya Stu. For ever i have used compromised, mis matched gear. But you know what? If ya can sail ok, who gives a ****? What ever you can put together to have a blast, go for it. We can't all be world beaters.
If your tall and skinny you might like it better with 430 better than the 460 anyway. Assuming the bend curve is combatible.
Being tall you need the boom higher in the first place so your more likely to find boom is connected to the mast in the area intended, and if your light you might like the softer master as it may suit your weight better.
