I didn't want to hijack the other thread about downhaul but it brought to mind something I've been wondering for a while.
I'm a relative beginner, and can't really 'feel' the difference between different setups too well, so rely on using recommended settings to make sure things aren't totally wrong. However, one thing that I'd like clarified is the lenth of mast / extension to use in different scenarios.
For example, if the luff marked on the sail is 440cm, then common sense tells me I should use a 430 mast and an extension set to 10cm.
If the sail has built in pulleys, should I use the above setup and be trying to get the pulley to touch (or as near as possible) the mastfoot? This ould obviously make it impossible to put on more than the 'recommended' downhaul, so should I be allowing a coulpe of extra cm on the extension to cater for this and allow for some adjustment?
And what if the sail just has a hole and I use a pulley - should I be making any allowances to cater for this, or has the sail manufacturer already done this when specifying the luff.
Reading about how just a cm or 2 can make such a big difference makes me think that I should know how to at least rig to the correct 'starting point'.
Cheers for any advice
Unfortunately, even as a beginner, you will have to learn to rig properly. Relying on numbers printed on dozens of manufacturer's equipment will not get it right. If you measure your 430 mast against 2 other 430 masts, there will be at least a couple cm difference.
The measurements may be correct if you are using exact same manufacturer's sail/mast/extension from the same year. Even then, being made of carbon, many masts and sails of the same production year/model will still differ. To me, the measurements are only useful as an equipment guide - so that a sail with a luff of 440 will mean I need to bring the 430 mast.
The key to downhaul is to downhaul until the second batten is centrally located on the mast. Most people do not downhaul enough and arguably, you cannot over-downhaul.
+1 what qldnacra says!! 
Easiest way is to study the sails of other experienced sailors who have the same sail, or at least the same brand and ask them why they downhaul to that point.
What is printed on the sail is usually only reliable as a guide. With most sails production tolerances and batch differences in the factory mean that luff length can vary by up to around a cm, sometimes more.
There is a 'standard' way of measuring downhaul length that most manufacturers seem to adhere to. It is the luff length when rigged measured to an imaginary line perpendicular to the mast base and level with the bottom of the downhaul pulleys.
measure your mast - as mentioned - to be sure what to put extension at and then
here is the MauiSails way:
