Responding to a request for written guidelines, I suggest checking out this site, it seems to have the best content and is very easy to read, with examples:
Further reading here:
The tricky thing is, the question is asking for guidelines applying to specifically percent-formatted numbers of text fields. Most guidelines tend to apply less specifically, stopping around the alignment and arguments for and against default text or hint text. I'm a fan of hint text! Hint text, of course being that default text that disappears when the cursor is placed within the field.
I would upvote obelia's answer except I'm new to this stackexchange site, and this is the first Q&A I've found. And I really like obelia's answer :)
But, must add my own useful and complete answer: As obelia said,
- having a % at the right of the entry field would be very useful
- disabling of the period/decimal point as input would add to the clarity.
But these are not perfect
- If you're left-aligning, as in your sketch-up, a percentage on the right isn't as pleasant. Bah, sometimes it feels awkward to force a right-aligned number entry on a form user, because textboxes default to left, and I bet 90% of number entry fields on the web and in paper are filled out left-aligned. oh well. Spreadsheets are my friend for right-alignment. :)
- If you want something more precise than 1%, like 0.5%, disabling the period is a no-go.
The solution I'd use is a default value in the field before focus, with a percentage sign at the end. Depending on implementation, this approach does not NEED to be hint text that disappears on-focus, it can be left in the field or removed at your discretion. And of course, assuming it is left in, then just do field validation when the field focus is lost. Then you can assume the person knew the proper style to input, and has entered what they want in the proper way. Plus, after the field validation, if they see a value that surprised them, they can edit again.
Anyway, it's unlikely a person who sees:
[ 0.0% ] ^v
[ 0.1 ] ^v
will ever be thinking "I want to enter 10 percent".