Start with your tuning guide to get your mast pre-bend close to where it should be for your weight. Then hoist the main with the boat fully rigged, and loaded, including main sheet blocks, with the sail on the mast head hook, and cunningham attached, but **WITH THE LUFF OUT OF THE TRACK**.
Compare your mast bend to the mainsail luff curve, when the down haul is near full on, and mainsheet blocks relatively tight. The curves in both the main and the mast should more or less match up, and your sail should flatten. If they match, and the sail flattens, it indicates that the combination of mast and sail has a broad range of tuning with the cunningham and blocks. If the sail has a big curve on the luff, looks baggy, either the mast requires more tuning to compensate, or the luff of the sail may require recutting.

While it is up this way, you may want to take a close look at what changing the mainsheet tension and cunningham do to interact with each other to create and decrease chord depth of the sail, and induce mast bend. It's kind of interesting.

Ther eis some valuable information here, as it applies to the A-Cats
http://www.usaca.info/index.php?option=com_content&view=article&id=229:landenberger-tuning-tips-mast-setup&catid=13:technical&Itemid=31