Interfacing with iOS frameworks is exempt. It's the third-party (not you, not the OS), pre-written software that matters.
My experience is that that is no where near 10% of the work. If it is for your specific custom app, you should line-item the work out.
In this case, if you are audited, they said they are looking for a "good-faith" estimate -- your source-code repository and history of checkins are a journal of this that is much better than anyone would ever have. I would recommend keeping the integration somewhat isolated so that it can easily be compared to the whole.
If you are using PhoneGap or some other application framework, then as the law is written, I think the whole thing is taxable. I think there is a good chance that this part may be amended -- if you are in MA, please write the DOR about this case (I already have).
Email rulesandregs@dor.state.ma.us -- just set up the situation and ask the question -- they will use it pretty much verbatim in their FAQ.