The voltage also changes depending on how heavily the device is being used, although I imagine the fluctuation would be less on the Mugen compared to the normal battery. dr_frost_dk made a queen beacon script to convert voltage to a percentage and display it, you can find it in his Battery Mod thread. I took that script modified it to include date and the battery percentage the N900 reports itself and I run it periodically, here is some sample output from yesterday, note at the time of the first reading I was connected via 3G and I wasn't for the second and also I was constantly playing music for almost all of the two hours between the readingsSo with his script based solely on the voltage my battery gain was 0.6% after two hours music playing, instead of the more realistic 17% decrease. You might still find it useful, but bear in mind that the voltage fluctuates a bit so it won't be terribly accurate.