Heh, howd you get that? most games it starts going down around 47-49%

I once managed to crank it to 100% and my citizens were still happy (80%!)

An interesting thing I noticed in 4C (haven't looked for it since then) is that there were points where if you went to a certain tax percentage by increasing the taxes, your morale was higher than if you got there by lowering it, even if the population never payed the lower (or higher) tax rate.

Yes ; I have this very often.
Something has to be done about those mood swings...
I understand the logic:
- turn n: happiness 60% with x colonists ; colony set to increase population by u% for the next turn
- turn n+1: happiness 20% because the colony increased in size ; now, unhappy, the colony is set to decrease in size.
- turn n+2: happiness 40% because the colony decreased in size because the colonists were unhappy (but did not yet desert you).

It's not logical for the colony to increase so much as to become that unhappy. The calculated growth rate should be computed so as to take into account the projected happiness. In the exemple above, at turn n, the computer should estimate what the next happiness is likely to be (estimates 20%) ; then it should reduce the growth rate to reach the "base happiness level" (where growth is supposed to stabilize ; I guess 40%) => linear interpolation and u is reduced by half : then, At turn n+1, the population reaches a stabilized point from where it should probably not move.

Implementating such a simple scheme could prevent a lot of mood swings...

If the game doesn't implement a "base happiness level", then it should still be possible to alert the player that "planet Y has seen a huge discontent arise", so that we could act before the planet desert... At any time where the hapiness level reaches level where desertion becomes a possibility, the turn summary should remind us of that very important fact! It's more important for me to know that planet X might desert than to know that planet Y has finished building an Industrial Sector and is now planning building a Discovery Sphere!!! The first is NOT in the turn summary while the second is there!

As for the relationship between happiness ans taxes (which certainly also includes pop size), sommething has to be done too so that a 1% change in tax rates does not bring a 30% change in mood rates. That's not how it works in the real world ; I'm afraid that the model currently used has too many modifiers and multipliers and that at some point (when you have too many morale boosts from ressources and technology and too many morale improvements), it becomes too sensitive.