You can't just look at levels in the soil as different environmental factors can increase or limit plant uptake. They are a guideline only and you ideally would have your hay tested to know how much that plant typically takes in. Your ag extension should know the range for hays grown in your area. Again just a guide but better than nothing and can help you choose between supplements if you are on either end of the range determined safe.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.