Some advice on identifying contributions to joint work
There are some useful approaches that can be taken to identifying your contributions to jointly authored work. The key is to maximize clarity and information.
Percentages are appropriate when you can state them accurately (e.g., "Jones did 50% of the work; I did 40%, and Frechette did 10%"). But when the division of labor is less distinct, descriptive prose is just as useful (e.g., "Jones took the lead on this project, contributing the majority of the work; I contributed the rest").
When contribution to joint work is to be sorted by type, that information is useful and should be supplied (e.g., "Jones had the idea for this project and designed the experiments; Frechette did the statistics; I wrote up the results").
It is a good idea to say explicitly when your co-authors are your graduate students and/or your dissertation supervisor, because that information helps to contextualize your own contribution. This is especially true for fields in which the order of authorship is viewed as important (e.g., "Jones and Frechette, my graduate students, are the first and second authors; I am the third author").
If you feel it is impossible or unhelpful to sort out who contributed what, just say that (e.g., "Jones, Frechette, and I collaborated fully on every aspect of this project").