Usually the rule of thumb is that the under tank heat source cover no more than 1/3 of the bottom of the enclosure. If yours covering 1/5 is giving you the right warm and cool side temps in the particular room in your home it's in then you're good to go. If you find it's not doing it's job then you either need to go to a bigger footprint on your heat source or up the room temps that affect your snake's enclosure. Also remember the lid is affecting how much heat stays in that enclosure for the snake to enjoy. Glass tanks with mesh lids tend to let heat and humidity very fast. You lose the heat you are putting in as well as dryer air in the enclosure caused by the humidity lose is hard to keep temps stable in.