All thermostats open over a range and 10-15C is typical, however, at idle the coolant flow rate is very low, so the t-stat doesn't have to be open very far for it to allow full flow to the radiator. Also, the difference in restiction between half open and full open is really not that much except at the very high flow rates that you only get at high revs.
In stop and go driving in hot weather whether the t-stat is open 1/4 or 3/4 is not going to make much difference in how much coolant flows into the radiator versus how much is recirculated back into the jacket via the bypass. It comes down to how much heat the radiator can transfer, which at a given ambient temperature is a function of air flow, and if the fans can't keep temperature in check then there's a very high likelyhood that the radiator has build up thermal resistance from deposits in the tubes.
There is a general misconception that overheating and be solved or even mitigated by installing a lower temperature thermostat. It's about as useful as putting a band-aid on a bullet wound.
The thermo-clutch responds to the temperature of the air exiting the radiator, and this usually correlates to a engine outlet temperature of about 100C in summer temperatures.