First, study the laws governing piece rate very carefully.
To establish the rate you need to conduct time trials - basically you count how many pieces you complete over a period of time. How many shirts can
you load in fifteen minutes, then how many shirts can someone else load in fifteen minutes; then you average the count and establish your piece rate. I used to do this fifteen years ago but I'm a little rusty so check the laws carefully.
Interesting article
Piece Rate Pay Design
Let me say that I believe there are better alternatives to paying piece rate in a screenprinting business (e.g. profit sharing programs); but I also think that most of the problems can be turned around with good management practices. Look up a recent thread posted by Out Da Box for some recommendations I made later in that thread.