NUM EMPTY LEVELS

About NUM_EMPTY_LEVELS
When you specify a number of empty levels, all your levels are shifted by that value.

So, if your layer works with a value of zero, when specifying a value of one, your level zero will be one level down in the sequence, which means that you have to double your level zero tile size degrees in that case - so that when you reach level one it will have the right value for your actual level zero.