Eq'ing is very subjective; so knowing how much to cut or boost pretty much boils down to a matter of taste. When using a graphic eq the frequencies are set for you, as is the bandwidth(see below), you cannot change them. All you can do with a graphic eq is boost or cut(by a given # of dB) in the frequencies preset in the equalizer.
I hate to throw another concept in the mix, but you have to consider bandwidth, or Q as it is sometimes referred to, which is defined as dB/octave. Draw an X and Y axis on a piece of paper. The Y axis represents dB (above the X axis is boosting, below the X axis is cutting), the X axis represents the freq (lower freq to the left of the Y axis, higher freq to the right)
Now draw a vertical line somwhere to the right of the Y axis and label this line
2K (2 Kiloherz).
Now look at this line, hopefully, you drew this with a pencil. Imagine the space that the width of this very thin line occupies along the X-axis. Not much.
The width of this line represents the range of frequencies(say 1.95kHz to 2.05kHz)that is affected by a boost or cut at a given frequency.
Now draw over that line with a wide-tip magic marker. This line should be much wider and represents a wider band (bandwidth-get it?) of frequencies(say 1.80kHz to 2.20kHz).
What this boils down to is that when you cut or boost at a given frequency(we'll call it the primary frequency) some of the neighboring frequencies(we'll call 'em secondary frequencies) come along for the ride. The range of secondary frequencies affected on either side of the primary frequency is determined by the bandwidth setting.
This is definitely an oversimplification and only partly describes what is happening as a given frequency is boosted or cut.
With a parametric eq you have much greater control as it allows you to set the frequencies to cut or boost, the actual boost or cut(dB), and the bandwidth(range of freq. affected by the boost or cut).
This allows you to cut problem freq. such as those at which hiss, harshness, or rumble may occur, or to boost freq. that need more presence in the mix.
I hope this helps you to somewhat understand eq, it was a very confusing concept to me until I began to think of it in mathematical terms.
dmc
[This message has been edited by dmcsilva (edited 11-17-1999).]