From the Breakpoints list, you can right-click on a breakpoint and select "Set Condition..."  You can then set whatever condition you would like to trigger the breakpoint.

Is it possible to add to the SF Debugger Watch List a request to Break (pause on location) if a variable is greater, equal, or less then some value?  For example, break (pause on line) if
rval > 1.e+16
rval < 0.0
rval = 0.0

If not, is this something that could be added in the future?

Frank