Lou,
I assume you're talking about your FJ here?
The technique for
dinghies is different than the one for keelboats. The wire inside the
jib luff takes over from the forestay and (tensioned correctly) will impart about 400lb onto a
rig of that size. Assuming it is correctly set up the spreader angle and deflection will cause the mast to bend to an amount that fits the shape of luff of the mainsail.
It isn't impossible to hoist the main first on a
dinghy, but it is harder. Generally speaking it is better to pull the
jib up first—easier and it means that the
rig is properly tensioned when you put the main up. Obviously, the considerations that apply to a keelboat (that it is hard/impossible to keep a balanced helm with the headsail alone up) don't apply: when you're doing this your boat is sat on its trolley just before you launch and you should be stood on the beach.
Unless you have an incredibly good reason (like, getting towed out a long way) you're going to want both sails up before you launch. Hoisting the mainsail in a 13ft
dinghy on the water is a giggle, but not in the right way! Depending on the type of jib-luff-wire tensioning system you have, hoisting the jib can require one person pull the forestay outwards to unload the
rig while the jib-luff-wire is tensioned. Obviously, this is impossible once you're afloat.
And, Lou, if you aren't talking about your FJ (or a different
dinghy) then all of what I've just written is totally wrong.