In my experience, the Federal Guidelines say Title I teachers must not
be used as substitute teachers. I can agree to covering for an hour while a sub is on the way, but not being used for a whole day to save a few dollars. To ignore this is to jeopardize the funding for the whole program.
Here is the link to your state's page on Title I: http://www.doe.in.gov/TitleI/welcome.html
I could not quickly find where it states the teachers must not be used for subbing. You may question your principal about what percentage to write on your form. There's a form, twice a year, where you must sign to state that you work 100% of your time (excluding duties that other teachers also cover) for Title I. That is turned in to the state and your district's compliance is expected. If you are still being expected to sub, you may locate your state worker (find in the website links) and ask for help.