This might be of use - and its used to see if you have enough fuel remaining so you can see if you need to divert:
http://www.csgnetwork.com/fuelconsumpgphcalc.html
If you want to do it yourself then...
Fuel required = (Consumption - using RPH × Flight Time) + Reserve + Start-up and Taxi
You can do this in flight to see if you will have enough fuel for the rest of the flight by comparing it to what you actually have on board.
As for other monitorings in cruise...if consists of checking fuel as mentioned above, checking engine read-outs (i.e. monitoring N1, N2, EGT, FF etc), checking weather reports, checking and establishing the diversion airport and calculate fuel required for that divert as well as checking weather reports for that diversion airport. Also, check and verify you understand the approach plates and ground charts of your destination and divert airport. That should keep you busy.
😉