If the summer wheels on your car are original Porsche wheels (not aftermarket), they should have a number on each side of the valve stem. One number is the width (like 7J) and the other number will be the offset (like 50). Check the front and back wheels to see if the offsets match. I've attached a chart I found on the forum a while back that may help. Looks like the front and backs for Boxster 17" are the same 55mm.
Also, here's another handy item someone had on the forum. It calculates the difference in and out between an original wheel and a new wheel based on width and offset.
http://www.1010tires.com/WheelOffsetCalculator.asp
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