There are many templates (I got one at the 1.JaVA site) they sometimes have the not used areas blanked out.
I simply draw over the edge to be sure. If you paint as far as the panellines go, then you are sure you are doing OK.
But another advise:
The XI series Spitfires aren't quite the easiest for skinning. I suggest you use the VIII to start with.
The IX series has bad wrapping around the fuselage, what makes making markings pretty hard. That is why so little people make skins for it.