Cooling conundrum

[e2e]

1990 200tqa 1b motor (10v turbo)
Hi I've just fitted an electric water pump to the car which involves milling
off the impeller from the standard water pump and removing the thermostat.
The new pump has a probe which I have inserted into the top hose by the
head, this controls the speed that the water is pumped around the motor via
a temperature adjustable controller.
I have a few concerns.
 The fan is going far too readily and is over cooling the water in the
radiator which makes said pump go too slowly and therefore the heater is now
terrible. Where is the sensor for the fan. I believe I need to change it to
kick in at a temperature above the max I set for the pump controller.
Is the current slow pumping likely to cook up the turbo.
What would the optimum temp be for this motor.
The dash warning is flashing at me while the analogue gauge is reading fine
(80 deg C). This can be either water level or temperature, am I assuming
correctly that the level is monitored solely off the bottom of the filler
bottle does disconnecting this bypass it or do I need to close that circuit.
The temp warning is which sensor and  what parameters trigger it off?
The warning comes on within 30 seconds of starting the car hot or cold.
Your thoughts always appreciated...
--
Sam Clarkson