My experience is that there's no simple formula. I've run simultaneous groups through very similar campaigns and I had to adjust for each group. So really it just came down to adjusting my XP budget until I hit a balance that I and the players enjoyed.
So with my current group the base calculation works decent (they're fun but not particularly effective). With my previous campaigns, one group most encounters were hard to deadly, for the other it was two or three hard and the rest mediums between long rests. Current group? One or two hard encounters, the a mix of low-to-high medium. With my "killer" group I'd sometimes increase their level for my calculation by 2 just to compensate.
I also use a slightly different encounter calculator that doesn't take into consideration the number of monsters. Ultimately though it's just a matter of trial and error. When doing the calculations, just increase the level or size of the group until you get the desired results.
There's a whole slew of advice on how to make encounters more difficult, but that's a different topic.
So with my current group the base calculation works decent (they're fun but not particularly effective). With my previous campaigns, one group most encounters were hard to deadly, for the other it was two or three hard and the rest mediums between long rests. Current group? One or two hard encounters, the a mix of low-to-high medium. With my "killer" group I'd sometimes increase their level for my calculation by 2 just to compensate.
I also use a slightly different encounter calculator that doesn't take into consideration the number of monsters. Ultimately though it's just a matter of trial and error. When doing the calculations, just increase the level or size of the group until you get the desired results.
There's a whole slew of advice on how to make encounters more difficult, but that's a different topic.