I have a form in which user insert inlet,outlet,change

the inlet -(change)=~ outlet

it means that the left hand side should be equal or + or - 2.5% of the outlet

i am confiused since i don't know how to make a function that reads the values the user entered and let the message pop up with the deviation value(the message sshould say you have entered a value out of the range your value should be between 5 and 10).