I just followed Philly teachers bucket Tek even though it's probably going to be way too much substrate I just wanted to make sure I got the Field capacity right. I'll just do a casing layer on top with the extra.
No, you must use very low to no nutrient casing.
However, coir is middle road nutrient. You won't do real well unless it is supplemented or you use plenty of rich spawn.
But mostly, don't use large amounts of coir for casing. The point of casing is to signal the mycelium that it has reached the extent of available nutrients, to store water, to act as an anchor for the fruit, to harbor beneficial organisms and to offer irregularities upon which to form primordia.
And the casing layer shoukd be placed after colonization