Hang on, 16 hours is only a part time amount of hours. I thought you wanted full time hours?
If you just ring in sick once or twice every couple of weeks, then you will not be very popular - with the rest of your colleagues because they have to take on the extra work from you not being there, and with the management because they have to try and find people at short notice and generally run a shop which is potentially far more hectic than is should be.
Just explain to your manager that you don't want to do more than 16 hours, and that you'd prefer less than that really. She will be within her rights though to say that in that case, why did you apply for a full time position in the first place when, when it comes to it you don't actually want full time hours? Tbh that's what I'd be asking if I were her. I used to work out the rota sometimes at Greggs - we had a set amount of hours for the shop per week which we couldn't go over, so I just put down everyone for their contracted hours, and then with the extra hours I'd divide them up as equally as I could between the people who wanted them.