Depends on the style youre after fella. Most Porsche wheels have a high offset (ie. D90's you see on Golf Mk2's are around ET52) with a 20mm adaptor you get an ET of 32 which sits them about the same position but obviously depends on the width of tyre rim you are substituting. So if you took a 6inch wide rim off in the above situation but put a 8inch back on your wheel is going to sit about 15mm further out towards your arch (sometimes termed poke as you go wider and wider). That make sense, you've added one inch to wheel width therefore the wheel is half inch wider each way from its centre line (plus another 3mm for change in ET from 52 to 32 due to the 20mm adaptor). So, you need to decide on the design of Porsche wheel you want to fit, compare its offset (ET) to your original, see how much tolerance you have and then decide on the size of adaptor you need to achieve the fit. There's some advertised on ebay and a guy on Edition 38 does custom, if you're MK5 you want 5x112 to 5x130, obviously the later being the most common Porsche PCD. Hope this starts to give you some pointers.....
