No this is too complicated! I never measure temperature of sand but slip my hand underneath occasionally to feel a soft warmth, almost imperceptible on a mild morning.
The thermostat setting of, say 20C is to give plants a range of suitable temperature which may go from 15 on a cold night to 25+ on a hot afternoon, at root level, and with much greater variation at leaf level. In Britain, where temperature is often on the low side, the more warmth the better, for rapid growth so if you set your thermostat to 25 you would find faster growth, but at a price…