What
Oluf says is correct. The trim setting is designed to assist the pilot in the event of an engine failure at V1 or faster. Remember, because the engine thrust line is high, a reduction of thrust (i.e. an engine fails) the nose will pitch up.
Also, it's not just weight but the distribution of weight that affects the trim setting. Full fuel and pax sitting in the back, trim is at 5.3 (or 5.4 if you want a margin against the config. warning). Lighter on fuel and pax up front, you'll need a higher trim number.
snorkboggler, yes the AFM is king, worked out by test pilots. Of course, if you'd prefer to take contradictory advice from us unknowns.....
Cheers,
Chips