Definitions:
1. Differential - the difference between the ON and OFF temp in a non-proportional controller. Rancos have an adjustable differential down to I think 2 degrees.
2. Setpoint - the desired temp of the heated area, or the highest temp allowed for the heated area.
You can use a Ranco. What will happen is when the Flexwatt temp reaches the setpoint, the voltage to the heater will turn OFF. It will not turn ON again until the temp falls to the setpoint minus the differential. Say the setpoint is 84 deg, and the differential is 3 deg. The heater will turn OFF when it hits 84 deg, and won't turn ON again until the temp falls to or just below 81 deg.
So you can see that the temp will cycle constantly between 84 and 81 deg in the above case, and the mechanical relay in the Ranco will click ON and OFF accordingly, alot. Not a big deal really.
Now with a proportional controller, the heater will stay at the setpoint plus or minus a reasonable error, usually very small (less than a deg), and if it does drift, it is very slow.
Some folks put a dimmer in-line with the heater (heater plugged into Ranco), to help fine-tune the system to minimize temp drift and relay cycling. This works well.
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Mark 
