What you should do is measure the temperature underneath the heat lamp where the monitors body will be located. If the temperature is 95-100F then I would just add a flat rock. If the temperature is too low then experiment with different rock heights until you get in that 95-100F Temperature range. make sure you disinfect any rocks that you use before putting them in your tank. Don't forget to add another hiding spot for the monitor, 1 on the warm side, and the other on the cooler side. If you have any other questions post them here and though I have limited experience with monitors I will do my best to help you. GL and keep us posted. Thats a very nice Nile you have there BTW