[Unclenick]

I don't think I've ever seen an example where an additional 5% off didn't cover pretty well for all potential sins among canister grade powders, except outright powder deterioration. Western used to recommend -15% for handgun starting load, and -10% for rifle. You will note some published starting loads are only 5% or 6% below maximum, and this can be because you don't want to let the powder fill get to be less than 60% of the space below the bullet (and many recommend 70% just for a margin). Going below 60% can cause some powders to ignite erratically and bounce both high and low.

The other consideration is climate. I don't live in a dry climate, and I don't test on very hot days. If you do live in a bone-dry climate and your powder sits a spell, even in an original container with a closed lid, it will eventually equilibrate with outside RH, and that increases powder burn rate, raising pressure. So, if you live in Arizona and are shooting on a very hot day and the powder has been in your dry air long enough to lose moisture, you can see where an extra 10% off might come from. Just don't go that low if it makes too much empty space in the case.