The pickup sensor will be about 50 degrees behind the pointer. So if you are going to run 35 degrees BTDC timing then put the first magnet at 15 degrees ATDC. You should be able to mock it up with your existing parts and verify where you want it before you order the shell from ATI.

Best Machine does a lot of these setups also so you could ask them if they have any ATI dampers or shells to sell. I don't know if they keep them in stock or not. You'll also need a special short version of the trigger pickup bracket. I make short ones for Best and FHO or you can build your own.