If you applied tape to your head, following the line of your natural hair and then placed the whole system on your head, you'd be able to lift the unattached areas and draw a line on to the underside of the lace base.
You'd just follow where the tape edge was and when you removed it, you'd have the exact shape of your bald area marked out onto the piece and then you'd cut around it.

Once you have a base thats the right shape, you can go back to taping the perimeter of the system and your old attaching method.

To get the positioning correct each time, place the untaped system on your head and when you have it in the correct position, flip the front lace up and with a makeup pencil, mark 2 dots on the underneath of the base and corresponding ones on your scalp.

You can then take the system off, tape up the perimeter and if you then match up those dots when attaching, it will always go back in just the right position.

Like this:
http://i1202.photobucket.com/albums/bb37...8e3bd3.jpg