I dont have a pic of the Big Reactor setup, but here is a Redstone NOR Latch.
The two swtiches pictured correspond to the two outputs. Put the "full" redstone input on the left, pressing that button will turn the right hand side off. Put the "empty" (<20%) sensor on the right. Giving that side a signal turns the right hand side on.
Additionally, just feed the redstone signal from the rhs back into the redstone port you have configured to turn the reactor on. (not too visible is a 2nd redstone torch on the backmost block mirroring the placement of the front torch).
So, while the reactor is <20% capacity, the redstone output from that redstone port will force the right hand side of the circuit to be on.
Once its above 20% (but below 80%) the latch will remember its last state (and so keep the reactor on), until
Once it hits or exceeds 80%, the left hand side of the circuit will activate, forcing the right hand side off, allowing the reactor to shut down.
Once below 80%, again there are no active outputs, but the circuit will remember its last state, (off) and allow the internal storage to drain to 20%.
repeat.
--
Additionally, if you have MFR, there are two ways to hook up a RedNet port.
The first way uses one RedNet port and a Rednet controller. You configure the Big Reactor RedNet port to output its energy level on one signal color, and to use a different signal color to control its control rod insertion %. Run a rednet cable to the rednet controller, where you choose the operation that simply sets the second signal as an output to be equal to the first input.
This means when the reactor is full, or close to full, the control rods will be fully inserted and the reactor will be off, or at least running really cold (and rather efficiently). As it empties the rods will retract and it will produce more power. The reactor will converge on a control rod insertion that matches your energy use. But which might not be at the best efficiency.
Or you can simply use two rednet ports and some rednet cable to achieve the same effect without using a controller at all.
With one port and a controller, you can use more signal lines and logical conditions in the controller to program a on at 20, off at 80 type behaviour too.