I find that propaganda is quite ineffective.

I had 4% approval on a planet at 30% propaganda ; pushing at 100%, my approval rating went up to, eh, 11%!

I have other such examples.

I don't know how propaganda is taken into account in the formula, but I feel that it is undervalued.

As a mettaer of fact, putting propaganda at 100% is akin to having a taxation rate of 0% for that planet.
I can understand why one would not want to do a one to one correspondance, but I believe that propaganda should work to reduce the effective taxation rate of the planet.

A possible fomrlua could be:

effective taxation rate = actual taxation rate x (1 - propaganda rate x conversion rate)

For example, whith a taxation rate of 70%, a propaganda rate of 40% and a conversion rate of 0.5 (this would be decided by design), the actual taxation rate would be 70x(1-0.4x0.5)=50%

There would still be a loss over a real taxation rate of 50% but I think things would work better than now, and one would really want to use propaganda in some cases (a newly seized system for example, which cannot support your high tax rate, at least at the beginning).

Yves