I did a post earlier on the ATP with delivery date control. If you have additional handling time you want to add to added extra buffer then you can use the issue margin. Here is a quick walk through of the effect of the extra issue margin on ATP.
Keep in mind where it’s going to look for the issue margin from the coverage group not the master plans. There are 3 possible places it could get it. 1. From the default coverage group set on the mater planning parameter 2. from the coverage group setup on the released item 3. from the coverage group set on the item coverage related to the released item. Remember it will search to see up item coverage, released item coverage group then default coverage group.