You could just write a custom function tailored to your use, IE:
GoalSeek( goal ; static )
x = goal / static
So if I fed in GoalSeek ( "10" ; "2" )
I would get "5" back.
All that "Goal Seek" really does in excel is, (assuming that A * B = C), divide the goal (answer) by the static number (A). You could make a more complex calculation based on what your implementation needs.
Also, if you wanted to run a quick calc in realtime, you could use the "data viewer" in filemaker pro advanced to run a quick calc (C / A) through.
Thanks so much for this feedback. I will be working with it today. I appreciate it!
If you're trying to mimic the functionality of what excel does, you could probably come up with a pretty simple script that does so in filemaker (set field, dialog box with a numeric input, etc..)