I don't think that the medical deduction threshold is accessible in any of the Money customization settings. This older thread:
http://social.microsoft.com/Forums/en-US/63aaf2d5-2fa5-4a77-9f58-0e1487377b81/2013-tax-rate-setting?forum=money
discusses which settings can be customized. The Sunriise project:
http://sunriise.sourceforge.net/
was developing a set of tools to access the Money database directly. Such a tool might enable a change to the medical deduction threshold, but I haven't seen any recent mention of that tool or its ongoing development.
Bill Becker