DD7 to me: "Mom, I always underestimate myself. So if I do something, I think I didn't do very well, even though I actually did pretty well. I tried to explain that to my friends, but I had to tell them what 'underestimate' means."

And she's right - for school projects requiring a self-assessment, she'll score herself 1-2 points lower than the teacher does, on a 5-point scale.

On the plus side, she has way more insight than I do - I don't think I'd have self-identified as a hard grader at 30, much less at 7. On the minus side, my lack of insight means I've never had much success changing my grading rubric.

DD is interested in bringing her self-estimation more in line with everyone else's standards, but I'm not sure what tools she needs to get there. Anyone have suggestions?

Wow that is great to hear! I would suggest that DD ask teachers for previous year's examples of A, B and C work.
Rubrics are wonderful also.
It's a real issue. There is also the issue of 'plotting the curve' of feeling of self judgement. For example, I'm in the process of writing my thoughts down for a handout. This morning I felt like it was terrible and a really bad idea. Now I'm feeling so glad I stuck with it. I knew at the time that with any 'art-like' project I always go through these emotional stages.

Gifted kids need peers so they can have a reliable reference group.
Best Wishes,
Grinity

My main suggestion would be to get her to see the whole thing as a game - the teacher's assessment of her isn't actually some absolute truth, any more than hers is. The value is not in being able to get the numerical score "right", but in being able to see what's good and what's not so good about her work. So how about a game in which (maybe in writing so as to have a record to look back on, if convenient, or else just in her head), she asks herself:

- what aspects of this pieces of work are good - correct, or just as I wanted them, whatever's appropriate for whatever it is. (Thought process: if I were explaining to someone who was really struggling how to do this, what would I point them at in my work as a good example of how to do it?)
- what aspects am I not satisfied with, and why? (It's really important to be able to identify weaknesses without incurring a responsibility to go and fix them! If that's difficult, you might suggest that she picks one aspect of this piece of work to be a focus area for doing better in the next, or something. But also, some things are not worth getting perfect, it's OK to decide that, and it's something I wish I'd learned earlier!)
- given that, what mark do I think the teacher should give me?
- and what mark do I think the teacher will give me?