So while all "real" 5e pretties do come with an XP value, that XP value isn't as easy to find when dealing with pretties converted from other editions and other games. The DMG (p 274) has a method where you find out a defensive rating and an offensive rating and average them together. My trick when using it was to put one finger on the defensive rating and the other on the offensive rating and then step one finger down, the other up (towards each other) one step at a time until they landed on the same line and that's how I found out the avg rating.
But sometimes that's too much of a hassle and you want to get a rougher & quicker estimate.
In Veins of the Earth there is a suggestion (on p 322) that when the PCs fight each other they get the other PC's HP as XP. That runs a little low, I thought, compared to many of the pretties in the MM.
So I divided every pretty's XP by their HP to find out the average. It's just under 22.35. Let's round that to 23 fnord. So multiplying their HP by 23 gives a rough estimate of XP?
This is the most true between CRs six to eight, but... if they're fighting a variety of pretties this might be a good enough rule of thumb? IDK. This experiment kinda proved to me that the full system on DMG p 274 might be worth it?