Lori, I believe the ratio is done to provide those who are using cubes as a substitute for long stem hay understand how much to feed. If you horse requires 18 lbs of long stem hay and you were going to substitute balance cubes, you only need to use 75% of the 18 lbs. Meaning 13.5 lbs of cubes would equal the calories of 18 lbs of typical grass hay.
The cubes have beet pulp in them in addition to the timothy and vit/minerals. Beet pulp has the same amount of caloric energy as oats, and so the cubes are a higher source of calories than long stem hay.
BUT....if your horse is only getting 13.5 lbs of cubes, and they eat them fairly quickly, then they are left with nothing for periods of time , which can lead to wood chewing, etc. People who only use cubes have to figure out how to stretch out the cubes.
If you want to substitute balance cubes for some of their hay, then figure out how much hay you're are substituting and multiply that by 75%. If you are just adding them to your horse's regular diet of 18 lbs of hay, you need to add the extra calories they receive from the cubes and determine if it's too much.
Dawn Wagstaff and Tipperary
Saline, MI 2003