I try and use commonsense.
No guides work for all horses. The recommended amount might be under the impression that your horse is only fed that, or only fed that and fed low nutritional hay/pasture, where as in reality horses often consume other feeds that provide nutrients that aren't accounted for.
I use this program sometimes called FeedXL. It's not perfect, and it does cost but basically it works what the horse is needed and % of what it gets from each food. My feed doesn't usually make up more than 20-30% of the RDI but I find that I reach it anyway through feeding chaff, hay and pasture in addition to the feed.
Right now I am feeding recommended as I am trying to put weight on my horse as well, but once she reaches her desired weight, or when the paddocks start growing proper grass, I imagine that I'll drop back a lot.
Many horses survive mainly off grass. It's not like they are in the wild where they have a huge range of feed sources and grasses, but it's not like they purely rely on our bags of feed.
Work out what your horse needs, what your feed offers and what else your horse eats to work out how much you need to use.