As a rule of thumb, the HP of a mon is preferably 1, 2 or 3 HP higher then the highest multiple of 16 (exception when you are quad-SR-weak, then only 1 more HP). That will provide you both the highest possible Leftovers regeneration per round and a buffer against residual damage. Granted, full on stall wars aren't common nowadays, but just in case some of your Pokemon might be grinded down by residual damage alone, it might end up living one more turn.
The most obvious example for the HP buffer would be switching SR weak monster in or using substitute. Let's take the subs example: Assuming a Sub-Gliscor does not have Protect but has to stall the opponent a bit. That would be the SubPass set from the analysis page last gen. It has 352 HP, which is a multiple of eigth (think multiple of 16 for Leftovers) and therefore only can sub six time before it only has exactly a quarter of its health left. But if you ask us, we recommend you to have one more HP, 353. After subbing six times, you are left with exactly a quarter of your health plus one HP, which allows you to sub one more time. Situational, but just in case.