[isoka]

You are doing it right since you are looking at the HP regen to kick in.
As you described, the mana regen update on client side is a bit broken.

You often get a wrong meditate tick on client side which corresponds to your "a couple seconds later while sitting, my mana will go up 12-13 ish, and then another second or two after that, it will go up 12 ish pts again".

Based on your description, you regain 12 mana every med tick. Sometimes, you get 13 as fake meditate tick, and when comes the med tick, your mana pool will be lowered by 1.
On your example, you get a fake meditate tick while sitting, so you have +12 extra mana on client side which you don't have on server side. When you cast cannibalize, server says that you should have +20 mana, so client gets updated and you seem to gain only 8 mana which is in fact 20 mana minus the 12 extra ones.

If you have jboots or any other instant click item, you can click it the moment you gain those fake mana points and you will see that your client will be updated with previous mana points. Once done, if you canni, you will gain 20 mana.