The next time you open MegaTune and look at your AFR Targets table, you'll find that it is scaled in AFR rather than Volts. The contents of the table will be garbage so if you already had values in here you'll need to re-create them, but from now on you can work in meaningful units.

Now, a lot of folks have reported problems with the default calibration, in that it can be a few tenths out in terms of the MS agreeing with the AEM. It seems that there is some variance from one AEM unit to the next. If you find yourself in this group, you can correct it by opening up the file file "lambdasensors.ini" which you'll find in the same directory as before. Locate the following section, and note that I've highlighted a certain pair of values in red:

#elif AEM_LINEAR
afr = { 9.72 + egoADC * 0.038666 }
lambda = { afr / 14.7 }
TargetAFR = { 9.72 + afrtarget * 0.038666 }
TargetLambda = { TargetAFR / 14.7 }

To alter the calibration, you need to change the 9.72s to something else. And this, sadly, is where the fudge factor comes into play. Some people have reported success in scaling the value up (to 10, for example) others report needing to insert lower values (9.5, in one instance). You're going to have to play with it and see what works. Just turn off EGO correction and eyeball the laptop display vs. the gauge at idle. Find a value that works for you.