Can you provide for people the placement of the probe from the herpstat? Is it on the surface, on the hot side / cold side, or is it suspended in air below the RHP or is it attached right under the surface of the RHP? What is the wattage of the RHP? Where are you measuring your temperatures at and are you using a thermometer or temp gun? RHPs, like CHEs radiate heat like a space heater but RHPs do not produce as hot radiant heat from their surface so it requires often a hotter surface temperature of the RHP to produce the desired temperature on the substrate or hide surface. For example, an RHP surface temperature of 115 degrees would not heat the substrate to 90 degrees from 2 feet above. So again, where is the probe at and where are you reading temperatures at?

Pro products uses heat loss / heat gain calculations to find the right size. As long as you provided accurate information, the size should be correct.