how far above grade is your deck?

overall dimensions of your deck?

again you will need to ask your building department, generally the concrete would be 6 inches above grade and your posts on top of it.

i dont like to run the tubes all the way into the ground, you wind up with a larger hole than you need and a loose fit for the posts until the earth settles back in around it, if you use the tubes at the top 15 inches of the hole then when you pour the concrete it will fill the entire hole and makes for a more stable pier.