For other adjustments, I just plug into the spreadsheet based on player equivalent level (see attached zip file) I got off of this site long ago and generally aim for hard fights. My current group works fine without level adjustment, in the past I simply entered the PCs as a level or two higher. So for example, I had a fight with a CR 16 storm giant quintessential and 3 fire giants for my group of 6 16th level PCs. Since they were going to have a limited number of fights between long rest I decided to go with (barely) deadly. It was a tough fight as expected.
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For other groups I've adjusted the PC level, adding a level or two to their actual level before plugging in numbers. I generally ignore the XP and normally do a mix of medium and deadly encounters or a couple of deadly fights like this example. I'm sure I could figure out the calculations for all of this if I didn't want to use a spreadsheet, I just find this easier.