What is helpful to me is seeing that friendly "standard CAP". It would be nice to see that for ground and naval attack ordinance. The database points in the right direction. But I often feel I am picking something that might be very rare. Wiki is a Godsend however.
Standard CAP loadout is very useful. A2G is much more problematic and is based on when the scenario is set. F-16C or F-15E are great examples, you open up the 'Ready' menu and you are faced with a stunning array of munitions, but most of them are older and have been replaced. I generally start with guidelines roughly as follows.
Base: if in a standard peacetime base location I err on the side of too much, at a forward base too little.
A2A: this is usually top priority for the air planners so baseline I start with lots and cut it back as I test. The standard numbers in the DB, generally 1-200 should be the basic 3 Day load for a Sqn. I don't know if that is accurate but I use it and it seems to pan out. Then I adjust for situation and story, which can be fairly drastic.
A2S: If not countering ships I don't include this at all, if I plan a major strike against a major naval force I add 2 loadouts (Harpoons and Kazoo's are expensive and big). Then adjust for any particular points in the scenario story.
SEAD: This is tricky, although I start with enough for 2 sorties per scenario day for SEAD specialized AC, I'll adjust as I test. Variable based on a bunch of stuff.
A2G: If major strikes are planned, 2 sorties per day to start with. Lots of dumb bombs (the base for adding guided kits), 20% of the newest type, 60% of the next oldest type, 20% special (penetrators, CBU, guided missiles) adjust for story.
Also, 20-25% on maintenance.
This is not set in stone and if you trawl through my scenarios you'll find vast divergences. Probably because I put a lot of stock in the storyline of the scenario, probably more than is needed.
I would say this is an art more than a science so you need to play with it and be flexible.
Oh and one other point: Testers will unlikely be happy with all your choices, that's good - it helps you tweak the loads and magazines, but it also points out the challenging bits and you decide if you leave it as that or not.