My boy eats just over a third of a bale per night when in and is on grass during the day. Hard to tell exactly as I give him ad lib so he always has some left in the morning. He does fine on this as it is year old hay so he does not pile weight on with it so long as he is kept exercised.
I work it out by giving her ad lib soaked hay, seeing how much she has eaten and then adjusting accordingly. At the moment she's on two slices a night because that is what she wants, any more and she just stands on it. By giving her soaked hay I don't need to worry about calories
I like J to have a bit left in the morning so give him 1/3 to 1/2 a small bale depending how big the bale is! I also hay the field druring the day for the sake of the outdoors girls, and that is a big square bale....I put out 4 small sections from that PM when I fetch Jai in then the same in the morning. Two outdoor girls are holding their weight very well, I hay them more to keep them busy than anything else as big one leans over the fence and buckles the top of it if she has nothing to keep her busy. J looks fit at the moment I think....but then thats only my own opinion and obviously I'm a bit biased!
Ours all get a round bale in the field (though today the girls and babies are without as they need a new bale!) and the 3 boys who come in at night get around 2 sections each to last them the night
Mine get ad lib haylage at night in their stables and ad lib dry hay in the paddocks during the day, I always make sure they have something to munch on !