I am not using the most up to date FC but I see the same behaviour.
I think the point of using a Std screw is that the other variables are fixed, and that saves time/effort by being filled in automagically.
You need to use the type 'none' then FC allows whatever variables.....
An alternative would be to make a table of your own custom sizes, and add it to the appropriate Mod directory in your set-up.
There was a thread about doing something similar fairly recently Vagalus or Bambuko IIRC.