Temperature to reach set point then hold off

Hello everyone 

I may be being mega simple but i cant seem to write the function block diagram for an immersion controller. using a  PT1000 sensor

I want to heat a tank up to 70 degrees regardless of how much lower the temp is, then once it hits 70 hold of the  reheat until there is a 10 degree drop in temperature. I have messed around with the differential threshold triggers etc but cant seem to figure it out. 

Any help would be gratefully appreciated 

Maxi