Here it helps, of course, that I'm not a very emotional person. If the person's judgments of how you feel are generally accurate, there's no need to use this - it's just for if someone is overestimating the amount of [insert negative emotion here] that you're feeling. I find it's good to use statements like this (that are absolutely honest, they have to be able to actually trust what you say), in the case of someone who is worrying that you are feeling some emotion or acting with some motivation when you're really not.
Anecdotally, this seems to be a common issue I've run into with family and friends on the spectrum, worrying that someone's angry when they're not. And small wonder. Caley's compared life when you have trouble reading emotions to a thunderstorm. Neurotypical people can see the dark clouds building and hear the rumblings of thunder in the distance and know that a storm's coming. For her, on the other hand, it's like lightning strikes in the middle of a perfectly clear day. In a word, it's scary and unpredictable.
To manage that, she tries to assume that people are angry too frequently, as she's learned that when you don't realize someone is angry when they really are, the situation can turn really bad really quickly. Better to err on the side of caution. But this caution can lead her to be afraid of non-existent emotional situations...which is where honestly telling her my emotional state comes in.
For example, in the case of Caley, I am very rarely annoyed or angry with her, but she worries that I am very frequently, due to her bad experiences with others. This method of honestly reflecting my emotional state works really well with her...and my other autistic friends, for that matter. The other interesting thing is, I've noticed that my own emotional state actually changes on analysis. My annoyance consistently ticks down a notch after I pause to try to give it a rating. I'm not lying to try to make her feel better about my emotional state - my emotional state (which is not generally terribly emotional...I think the highest rating I've ever given was a 3/10) calms upon the introspection required to give the rating. So the method really works well for everyone all around.
Of course, here's where I'm going to remind you all that I'm not a professional. This is just something that's worked well for Caley and me, as well as for when I'm interacting with friends on the spectrum. If you think it might work for you, I suggest trying it out - it's been great for us, and from what I can tell there's nothing to lose and a lot of emotional relief to be gained.