Adjusting your chain

This might be a problem, you shouldn't really notice any difference, certainly not "much more". The chain isn't really stretching per se, what is commonly called chain stratch is really more from wear and tear on the side plates where the pins go thru, over time these holes become larger allowing the plate to move in relation to the pin, or you could say that the pin is no longer in the center of the hole, but now has room to go to one side. Sort of like a pencil in a coffee cup. The size of the hole should not change from hot to cold and thus your slack shouldn't (yes I know technically the metal will change with temperature, but not enough to make for a "much more" change).

And I have yet to see an owner's manual that says to adjust the chain slack while seated on the bike. If you adjust it to spec then, it should be too loose in reality. The reason you have some slac in your chain is to accomodate the swingarm movement when you sit on the bike, or the suspension absorbs a bump. Otherwise a tighter chain would be more efficient, but wouldn't allow sufficient movement of your suspension.