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How do I account for differences in weight/bearing surface, other words, what's the margin of error for that?
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It all depends on the cartridge being loaded. Some cartridges are more forgiving than others. The smaller cartidges usually run at higher pressures and therefore have a smaller margin of error. With Lead bullets, the main thing I concentrate on is the weight. Then from there, I will look at the style of bullet and if it is remotely close, then I will use that data. I start low and work up. Use common sense. Don't use 150 grn round nose data when loading 148 grn wadcutters (because you obviously seat the wadcutters way deeper than the RN design.) Obviously the seating depth has to be taken into account to...which you already knew. Remember....squib loads are only bad if you don't realize you have one and follow it up with another shot.
As far as weight...it depends on the cartridge. Like I said, the smaller cartridges have less margin of error. For 9mm (closest thing to 380 that I load), I may use load data for a given bullet if the bullet I am using is no more than 3 or 4 grains over the specified weight. It is generally safer to use load data for a heavier bullet when actually using a lighter bullet (but you must again watch out for squibs). Say for example I have a bullet that weighs in at 140 grns, I would use the start load for 147 grn bullet if that was the closest load I could find....and work it up from there. The opposite is NOT safe. I will reiterate that seating depth is very crucial too. Pay attention to seating depth. Compare seating depths on various loads. You will get then hang of it. If you are worried, you can purchase a program called Quickload. This can help develop your own loads and seems to be very precise. If you go this route, still work up your loads unless you trust a computer program with your safety and well-being. Good luck!