I’m going to have to side with [MENTION=22779]Hussar[/MENTION] on this one. In my experience running ToD and SKT i’ve not had trouble hitting the PCs with legendary creatures. The problem is the creatures just don’t have the longevity to ride out the fight to a point where the players are getting nervous about their characters. In the final battle with Iymrith in SKT I had to fudge her an extra 200 hp so she could at least keep some dignity!
I don’t know how much damage the PCs can deliver per round but going forward I’m definitely going to give monsters max hp and legendary creatures maybe 2x max hp. I’ll feel free to fudge as the combat goes and hopefully I’ll find some number that works.
Is your group more than 4 players? Do they use standard ability score generation? Do they use feats? Do they have magic items? Are they experienced players? Do their character builds work well together? Do they have more than 1 long rest per 6-8 encounters?
Any combination of the above is going to affect what you need to throw at PCs to challenge them. Unlike a computer game, the developers don't have control over your group or your effectiveness as a DM at implementing smart tactics for the monsters.
This thread has devolved ... do you have any suggestions about how to scale up encounters to get to the difficulty level you seek? I gave a page or so of suggestions several pages back, is there anything other than adding HP to make encounters tougher?