Rule of thumb. :

-1- When you need to let go of more mainsheet then an arms length (not adjusting your grip) then crank on some more downhaul till you can suffice with an arms length (or less)

-2- If cranking on the downhaul is too hard to do then head up into the wind a little and crank on the mainsheet after which your crew pulls the downhaul tight. Then release some mainsheet and bare off. The downhaul will now be tensioned much more then you could achieve without a tight mainsheet.

-3- If all this is still not enough then let the mainsheet traveller out some.

When to lift the boards on a cat is very dependent on the design and on how you like to sail it.

Wouter