A 5" Livingstoni will eat a LOT of flake food. 
So how much is enough?
I have been told and read on "this" forum that you essentially want to starve your fish, no more than once a day and only enough that lasts 2 / 3 minutes in the tank before it is all gone.
I have been known to over feed my fish, water gets cloudy, have to do lots a WC's. I have since cut way back on the amount of food I feed my fish, now this occurs with my pleco..
Just goes to show you what little I really know about this hobby...
I seem to learn more and more each time I login to the MCA...
THIS IS A GREAT FORUM - THANK YOU FOR ALL YOUR HELP ! What a GREAT wealth of knowledge...