To adjust the spacing to either increase or decrease from the last line of the footer to the edge of the bottom of the receipt, here are the following methods.
The screenshot below will be a reference to the syntaxes we are about to use. We will use the Reports key [2], shown as the 2nd button to the right of the Home key [1] in the Menu Icons (1) section, and the Numeric Keypad (3).

Increase Spacing
On the keypad, enter the following:
9 9 9 2 + Reports key
Decrease Spacing
On the keypad, enter the following:
9 9 9 1 + Reports key
Both functions will go by an increment of 1. You may need to perform either functions multiple times until the desired spacing on the bottom of the receipt.