[Motorola Droid X2] RealPariah:
The stats can easily drift if you don't flash ROMs with your phone at 100% battery, hence why sometimes the calibration is needed [be sure to wipe stats when at 100% [either via app or recovery], best thing to do is fully discharge battery until auto off occurs and then fully charge with AC Wall Adapter and charge only screen, then when it hits 100% wait a few min, restart to OS to be sure it also says 100% [if not wait until it does and wait about 20 min longer] then restart to recovery and flash away [you could also wipe battery stats as part of flash if you do as stated before flashing]. I usually set it up so I fully discharge the phone [restarting to be sure a couple to few times] and then charge with phone off and charge only screen overnight and flash my ROM first thing in the morning right before I unplug it and unplug once I reach the point I am ready to restart device and do initial boot after flashing. Further details below...

Basically charge to 100% [or as absolutely close as you can get it [AC Wall charger is best unless you REALLY are forced to do it via USB and is best to charge via the charge only screen [phone is powered off and not booted in to the OS and all you normally see is just a battery filling on screen [and is fastest way to charge battery]]. Wait an additional 20 - 30 min after it registers 100% [this is to be sure the battery is absolutely topped off essentially] and I will generally do a restart as sometimes the battery may come back to less than 100% on a restart if your phone is not judging the battery right and is in need of calibration. If it does not register after restart wait until it hits 100% and wait the additional 20 min [you can cut out the initial 20 min wait if you want to do the restart to verify just wait the 20 min once you feel sure battery is as topped off as you can get it], then after wiping restart from recovery and unplug. Now be sure to drain the battery until auto shutoff [either stream media if in a hurry or through general usage. Either way wait until auto shut off.]. I usually will power the device back on and be sure it is not going to get back to the OS [if it even gets to boot logo I wait a few seconds and power it on again to be sure all I get is the boot vibrate on my device [some don't have this, but usually it's tablets almost all phones [and definitely both Motorola and Samsung do this]. I then charge it to full [again AC wall charger recommended as above and again with charge only, but as stated if you cant live with phone off or whatever you can do it with OS running as well. You are free to go as you wish after this second full charge really though if you do a couple more [dont have to be insane] it can help ensure the statistics get a good start.

As I believe I stated above the best way to avoid calibration as long as possible is to charge phone to 100% via phone off screen [with AC Adapter and wait the extra 20 min after it registers full before you flash [I will sometimes take it a step further, leave it plugged in while flashing my install zips and then once I go to restart system for the initial boot after ROM flash I will unplug the charger from the phone.

Hope this helps :)