Because the boom angles upward toward the back of the boat, pulling on more down-haul also has the effect of tensioning the foot tension of the sail. When we were first sailing the 20's, some guys were pulling on the outhaul too much in the rush to get sorted at the leeward gate, then jumping out on the wire and hammering the downhaul which over tensioned the outhaul and pulled the bottom of the sail out of the mast.
To set the correct length of this limiter line, rig the sail with loose out-haul, pull on a shed load of down haul, then pull on out-haul until it is snug. This is then the position of max out-haul and the line can be adjusted accordingly.
In a decent breeze, letting off the down-haul also eases the outhaul enough. Only in really light stuff did we use the actual control on the boom the add more camber.

Paul