Thanks for your time and work! Would you be able to calculate for me maybe 3 hard to deadly encounters within a day? I don't think I'll get more out of a 1-shot. I'm thinking something along the lines of:I'd say treat it as if they were about 4th level characters or so for an XP budget.
I based that guess on taking the total HP for a group of 4 CR 6-7 monsters (160 hp x 4 = 640), then dividing it by 3 (est. how long a combat is supposed to last on average), then dividing it by 8 (number of "average" encounters during a day). That gave me 26 - so I checked to see what CR monster does around that damage per round - which is CR 3). Then gave them a bump for being PCs in this case (and having access to HD for healing). What that basically tells me is supposedly how long they'd last before an adventuring day wipes them out.
I echo @Paul Farquhar's estimate (treating them as 9th - 11th level PCs for encounter building seems about right).If I want to have my 4-5 players be CR 6-7 monsters for a 1-shot (an Oni, a Shadow Dancer etc.), what CR rating/budget would the actual monsters be? What creatures could they actually take on? How would one calculate that? I am a Maths Numpty. Cheers.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.