It can be very difficult due to the fact that the frequency change with each click can vary between .2 and .4 (it also rockets through the frequencies if you hold the mouse button, which isn't bad for progressing to higher/lower numbers before using single clicks to fine tune). So I find it best to have *Cockpit ToolTips displayed so the frequency is showing in the ToolTip as I'm rotating the knob, and I can eventually settle on the correct frequency (quite a few left and right clicks as it jumps below/above the frequency before nailing it).

*Menu > Options > Settings > Display > Aircraft (Tab) > Checkmark (tick the box) Show cockpit ToolTips