Vcc calibration / values changed between 20180403 and 20180511 builds

[netraam3371]

Hi all,
Two questions, related to Vcc
First, I noticed that the Vcc reported seems to have been changed somewhere between 20180403 and 20180511. I noticed this when going back to an earlier version when trying to get an MH-Z19B sensor to have ABC re-enabled. With newer firmware, the Vcc reported is around 3.25, while older firmware reported it as 2.99. In reality, both values are not entirely correct (should be around 3.19). And, another esp-12f currently reports Vcc as 3.52 while in reality the value is more like 3.30.

So, two questions:
1. Did the calculation change in recent build?
2. Is there a way to "calibrate" the readings so they can be more meaningful?

Best, Maarten