Another approach could be to normalize everything against your best skill. So rather than saying "I'm this good at X", you're saying "I'm about half as good at

Another approach could be to normalize everything against your best skill. So rather than saying "I'm this good at X", you're saying "I'm about half as good at X as I am at Y." I think the tricky bit would be making your intentions obvious. I actually took a similar approach on my résumé: I broke my skills up into categories relative to each other rather than vying for some absolute quantification.