Note that I've only been doing this for a few weeks, but I always weigh what a given disk is dispensing at the time I'm actually going to reload (and check charge weights ever 10-15 rounds anyway as part of quality assurance). Lee says and common sense agrees that things like temperature and relative humidity will vary and this will change the amount dispensed by a given disk. Also different lots of the same powder may vary in the way they meter through a powder dispenser.
I glance at the Lee table for "ball park" data on which disk to try for a starting charge, but after that it's trial and error, moving up one disk or disk cavity at a time - and already I am working more from my notes than anything else. But each time checking what that day's results will be before I start reloading.
HP-38 meters well in the Auto Disk. Your "error" of .3 grains on that one disk sounds typical. If you really really want a specific charge and that amount is "straddled" by two consecutive disk cavities, consider the Adjustable Charge Bar (for charges of 3 grains and up). There's also the disk-stacking option, an extra you can buy from Lee. Or modify one of the cavities by partially filling it with something (there are posters on this forum who modify their disks this way - that way you can have a dedicated disk or cavity for a specific favorite charge - search this forum for details).
