Companion Spirit Gems Not Showing Correct Spirit Reservation

I have 103 available Spirit: 60 from the campaign + 43 from my body Armour.

Companion Reservation shown on gem states = 42.3
Herald of Blood Reservation = 30

When both are activated, it shows 29 spirit reservation is still available.

However, it should be the following per gems description:
103-42.3-30 = 30.7 remaining spirit.
* No support gems using additional spirit is slotted.

Either the calculation is incorrect, or Companion reservation as shown on the gem is incorrect.

Companions reserve a percentage of your total spirit, not a static amount.
Math says the result of yours is about 29-30 depending on rounding, so it seems correct.