Huh. Interesting. While leveling up, how do you decide when to go up in difficulty level? Do you typically start on normal or expert or what? Do you drop back down for known tough boss fights?
I'm not the greatest font, as I only have two HC toons: crusader and witch doctor. But I'll give it a go with my methods.
I play the entire campaign on normal. I did a search for rough estimates on levels and damage, but at the end of the day, it comes down to feel. I was very meticulous in Act 1, fully exploring and doing all side crawls for XP. Crafting pays off big time in the earlier levels.
My WD was killing it by the end of Act 2 on normal, but I chugged away on normal, knowing that my gear would suck again and I'd have to wait for the next wave of upgrades. But at the same time I just started charging through the campaign. I was in the 30s when I beat Diablo. Probably forties or low fifties for Maltheal. In softcore, I would always get up to 70 for Maltheal, but my mindset is different for HC.
After I beat the campaign, I almost immediately stepped up to hard due to amount of faceroll I was having in Normal. I think I made it most of the way to 70, if not all the way. Because I don't have a crazy amount of Paragon levels, I still had to gear up for expert. And I play it super safe with my HC toons. My WD is very likely ready for master, but like I posted earlier, I just need a few mats and expert is better for that due to the speed I can clear it on.
My difficulty check is two-fold. First one is just going to to the festering woods in adventure mode and trying out the mobs. If I feel good about that, I'll stick around for a bit and do bounties and all that. After I feel pretty good about how the toon handles bounties, I'll try a rift.
Depending on how the rift goes, I'll stay at that difficulty or step back down for a bit until I get some better gear. My crusader jumped between T1 and Master about three times before I finally stayed at T1. T2 is just within my reach, but those scary moments I had during a T2 rift were pulse-pounding. Luckily the horsey and my kind of ridiculous healing stat saved my bacon each time.
And after entering a new difficulty, I avoid the Kill Maltheal/Adria/Izual bounties until I feel really good about my ability to take him on. In fact, I run a lot of Act I after I first get into a new difficulty. Slowly expanding from there. Act V is still a whole different beast from the rest of the game in each difficulty, at least for me.