Take a look in the samples folder--C:\DirectRT\samples\6 v2006\gnat. That sample might help you with what you are looking for--that is, creating a fixed window with a spacebar/no spacebar situation. You maybe just need to use a min-max value of 0-500 in your situation. The gnat sample is also documented in the user's guide and online here:
http://www.empirisoft.com/directrt/h...e_as_valid.htm
Let me know if that still doesn't help.