Another way to look at the formula we derived above is
DCF = RR * HFL * VFL
Note, though, that your refresh rate cannot be greater than the maximum vertical sync frequency of your monitor. Thus, for any given monitor at a given dot clock, there is a minimum product of frame lengths below which you can't force it.
In choosing your settings, remember: if you set RR too low, you will get mugged by screen flicker. Keep it above 60Hz. 72Hz is the VESA ergonomic standard. 120Hz is the flicker rate of fluorescent lights; if you're sensitive to those, you need to keep it above that.
Flicker is very eye-fatiguing, though human eyes are adaptable and peoples' tolerance for it varies widely. If you face your monitor at a 90% viewing angle, are using a dark background and a good contrasting color for foreground, and stick with low to medium intensity, you *may* be comfortable at as little as 45Hz.
The acid test is this: open a xterm with pure white back-ground and black
foreground using xterm -bg white -fg black and make it so large as
to cover the entire viewable area. Now turn your monitor's intensity to 3/4 of
its maximum setting, and turn your face away from the monitor. Try peeking at
your monitor sideways (bringing the more sensitive peripheral-vision cells into
play). If you don't sense any flicker or if you feel the flickering is
tolerable, then that refresh rate is fine with you. Otherwise you better
configure a higher refresh rate, because that semi-invisible flicker is going
to fatigue your eyes like crazy and give you headaches, even if the screen
looks OK to normal vision.
For interlaced modes, the amount of flicker depends on more factors such as the current vertical resolution and the actual screen contents. So just experiment. You won't want to go much below about 85Hz half frame rate, though.
So let's say you've picked a minimum acceptable refresh rate. In choosing your HFL and VFL, you'll have some room for maneuver.