How much Maelstrom does the Elemental Shaman mastery give?

I was initially under the impression that maelstrom generated from overload procs was 50% for all spells. However, I tested this on the training dummies in Dazar'alor and found this is not the case.

The following test results are from patch 8.0.1 (October 9th, 2018):

• Lightning bolt overload and high voltage procs generate 3 maelstrom (37.5% of base).
• Chain lightning overload and high voltage procs generate 2 maelstrom (50% of base).
• Lava burst overload procs generate 4 maelstrom (40% of base).

The maelstrom granted is different for each spell, which explains why the mastery description is vaguely worded.