Its the eternal question, how and when to feed, too much and you can overfeed them too little and they go elsewhere...
On rivers, my own strategy is to fish for bites, scaling tackle down whilst still remaining sensible and introducing a couple of small balls (golfball size) of groundbait from a of packet something with more attractant than feed plus a few hook bait samples at the start. If get bites then I try and increase the feed rate to suit.
It also sounds daft to say this but if you know there are fish around - ie from previous catches you may feed more from the off.
I keep tricking bait in - in colder months, I have found that there are feeding windows where, for a period where the fish will suddenly 'come on' to feed and equally suddenly stop - light levels, temperatures etc be damned.
I say this having spent two sessions at my local river over the weekend without a bite
mg: I thought that I fished well, its just that the fish weren't in the mood - ever the optimist
I'm constantly working on accuracy of feeding, I remember Bob Nudd saying that feeding was the most important part of fishing and along side enjoying yourself, I believe this to be true.