Matt:
As you know, the EM10 has a dial on its face that you turn to adjust the sensitivity. You can do one of two things to have the green LED illuminate rather than the red LEDs on either side.
1) adjust the intensity of the light that hits the sensor, or
2) adjust the dial on the EM10.
A couple of points to remember.
a) readings need to be done with the safelight in your darkroom turned off, and
b) the EM10 is not linear, and appears to need light intensities within a certain range to function well.
The consequence of the latter point is that if the "perfect" print you do is done with either very bright light (and a very short exposure) or very dim light (and a very long exposure), you may find that it is either difficult to obtain any green, or that the green will be obtained with the EM10 dial set near one of its extremes, thus potentially rendering the result unreliable.
If possible, you want to use a setting that is somewhere in the middle of the scale.
Then, once you determine the setting you are going to use, you set that number on the EM10, put your new negative into your enlarger and adjust magnification and focus to where you want it. Then you take the negative out and measure and adjust the light intensity (using the aperture on the enlarging lens) so that the EM10 goes green.
There is one further complication to consider, and I don't know whether I have arrived upon a satisfactory answer to it. If, like me, you use variable contrast materials, and a colour head, it may be necessary to make both the calibrating readings with the colour head set to the same filtration numbers. I don't know what the spectral sensitivity of the EM10 might be, but I expect that it is relevant.
For that reason, I've been leaning to using the filtration settings on my colour head that match filter grade "2" on the paper information. I also use the numbers on the scale which does its best to give similar exposure times for different grades.
I would bypass the filters and use the white light instead, except I find that my enlarger is too bright for the EM10 when I do that.
Hope this helps.
Here is the link on Ilford's site to the instructions:
http://www.ilfordphoto.com/Webfiles/2006821125512773.pdf
Matt