I think this problem has been around for several releases
now, which is astounding... The install program asks you to
select a monitor from the list or enter custom data, and
I've never owned a monitor on the list so I always choose
custom.  Then it asks for the vertical and horizontal
frequencies, giving a list of predefined choices and says
that I can give a specific range if I want.  Yet there does
not appear to be any way whatsoever to enter such a range!
There's no option in the menu for it, and the program just
ignores typed numbers, so what the heck are we supposed to
do?  I've always had to just choose standard VGA and then
do the X setup later with one of those bogus setup tools
that have never really worked anyway.  Grrr.