Pacing is easy - if I feel significant vertigo, or one side of my body stops responding, I should have stopped everything 2 hours ago.
Not that I do things for hours you understand, I do little sub tasks, preparation etc, and split tasks into multiple bits, resting in between, but I still frequently over do it, because if I paid much attention to more minor warnings, like dizziness, fluey type symptoms, difficulty standing or moving without collisions, then I'd never get anything done.
So my pacing currently is just try and avoid doing stuff at a level that would leave me unable to manage the minimum I need to do to continue living. e.g. 2 days ago I didn't get a needed bath because the vertigo was significant, and trying to get a bath whilst everything is spinning and sliding in 3 dimensions, when I literally can't trust my body to know where it is, or what way up it is, is no longer entertaining, and often has negative consequences.
Sometimes I push, sometimes this, experience has taught me, is inadvisable.
All weighted by the importance, and urgency, of what needs doing.
As well as my ability to actually start.
ETA - I suspect that the problem isn't with how I do things, over the years my ability to pace has become okay, as far as I know I know all the 'tricks'.
The problem is that even using everything I know I simply do not have enough resources to live and stay out of PEM, so it's become a matter of managing the PEM, trying to keep it from getting completely out of control, rather than trying to avoid PEM full stop.
Of course bonus extras, like my GP, the DWP, having to deal with changes caused by the pandemic, and my sister (who announced she is coming down and staying a £45 taxi ride away) - all do not help when I'm struggling anyway.
ETAA - I suppose, in the spirit the of the thread title;
Minor warning signs include;
Dizziness, headache, lower body flueiness, walking into walls, door frames, etc., IBS, inability to coordinate hands (e.g. to wash/dry things, get plates out of cupboard in a 'proper' manner), minor (for me) cognitive issues (who needs a brain for most tasks anyway lol) - safe to complete small tasks, as needed, until escalation.
More significant warning signs;
Vertigo, some types of gastric sensations that resemble poisoning, falling into walls/ door frames (hitting the obstacle at a lower height with same body part, which is suggestive that I was falling and only checked by it, even if I wasn't aware I was falling), escalating and relentless pain levels at rest, some signs of immune activation distinct from normal flueiness, cognitive issues beyond normal, virtually instant fatigability with trivial things - time to lie down but if I am engaged in a very short task, like getting a drink, or making a simple sandwich, then stop, wait it out and, when it decreases, that's fine to complete. Repeated/multiple occurrences are bad and should prompt a day or 5 of rest.
Urgent warning signs;
Vertigo at rest, dizziness and vertigo that increases when standing, even if given time to stabilise, lack of control and cooperation from more than 1 limb (normally it's my left side that decides to go on strike), rage, for no obvious reason, different type of poisoning sensations, or escalation of the other type. - stop immediately and rest, normally bed rest.
This can go one of 2 ways, either a few hours bed rest and drugs, followed by a few days of no non essential stuff will stabilise it, or it's an indicator that a crash is imminent and it's too late to stop it.
Basically with any of these more serious warning signs I am going to be unable to complete even simple tasks so...might as well rest, wait, and see if things improve, as if I don't my body is going to make life pretty miserable anyway and i'm probably going to not just not achieve anything but have to make good and redo anything I try.
Further refinement of this stage is needed but TBH I rarely push things this far, it mainly happens by 'accident', as a result of multiple smaller infractions, and I'm not generally in a state to make close observations.