Shingles are attached in a triple fashion. How do I cut the shingles with a circular saw to attach the ledger without hitting the nails that can not be seen from the outside?

Please help! The more I research building a deck the more I get confused!! I am in the beginning stages of building a freestanding deck and getting more and more confused! The deck will be in total be 32’ long - one section will be 16’x18’ and the other (attached) section will be 16’x12’. I had decided to use and have purchased Dek-Blocks. I originally thought I just had to use one for every place that I would have poured a footing but after visiting their website today I realized it was wrong. It looks like I’m going to have to purchase a lot more blocks! I thought I read a posts here where someone (Lefty maybe?) recommended putting the pier blocks in 2 inches of cement but for the life of me I couldn’t find it when I went back to look…..of course! I’m in Michigan so frost will be an issue. I opted for the Dek-Blocks instead of the traditional footings because at its furthest point the deck will be out of the ground about 3.5’ and since I’m building the deck from lumber I currently have using the blocks made my 4x4’s go a little further. So I guess I have two questions. 1. Do I really need to use so many Dek-Blocks and 2. Do you need to cement them in the ground as well? I had already dug out a square section about 4”-5” deep as I read (somewhere here) but now I’m questioning if I should have done that because of the frost! I’m probably making this process much more difficult then it needs to be…..sorry! Amy