FIELD VALIDATION - NEED TO VALIDATE $$ AMOUNT ENTERED INTO FIELD

Here is where I could use some help...I want a signer to have the option to enter an "other" deposit amount On the form, they can select (via a radio button group) 100% or 50% of the total amount as the figures are auto populated.  There is a 3rd option in which the customer can enter a manual amount which should be between the 50 and 100% amountt.  Currently, with no validation - the signer sometimes enters an amount less than the 50% - which needs to be prevented.  Can someone assist on what that validation would look like?