how is tax income calculated?

There is a flat rate per population.  There are some planet bonuses that may add to tax income, and your capital receives extra tax income.   Planets with insufficient population upgrades suffer an underdevelopment penalty (I'm not sure whether this penalty is subject to modifiers, but in any case you want to get rid of it asap because underdevelopment costs a small fortune).

The modifier is loyalty.  Whatever tax income you would get is multiplied by loyalty.  50% loyalty means only 50% tax income.  This is where trade is useful, since it ignores loyalty and provides higher returns for big empires, but for small empires it's tax income all the way!

http://turbotax.intuit.com/ 

Does that help?

Tax income, not income tax

I think the OP was wondering what that flat rate was if alligence is at 100%. I know regular asteroids at 100% give 0.6 credits, so I would guess its something like 35 population gives you 1 credit. But I don't know exactly how much each population "unit" gives exactly.

Rereading the original post, I noticed this:

Buy two immediately upon acquiring the planet (asteroids only need 1).  If the loyalty is above 60% and its population is nearly maximum, it's worthwhile to buy the next level of population upgrade.  That's how I do it.

Unless this info is seriously outdated, I think that each unit of population = 0.025 credits/sec, such that 40 population = 1 credit/sec (this is without allegiance, which acts as a simple multiplier to the final tax amount).

https://www.sinsofasolarempire.wikia.com/wiki/Resources

Darvin3's method is right on the mark, getting two population upgrades will stop the underdevelopment tax for all planets (except asteroids and dead asteroids). If time and conditions permit, try and delay the actual colonization until you have enough credits / resources for both of those upgrades so as to reduce the credit losses (which, in the early game, is quite a significant reduction in your credit flow).

You may not always want to go for population upgrades first; if it's late in the game and you've acquired a new planet from an enemy force, you may want to go for emergency facilities / tactical or logistical upgrades first to hunker down from an enemy counter-attack.

I'll think of some more tactics later...

You should never buy emergency facilities first.  Planets begin with about 250 health and take some time to reach the 1500 initial cap, so really it should only be your third or fourth upgrade at soonest.  Tactical upgrades are the only ones I'd prioritize ahead of population upgrades under any circumstances.

Thanks for your informnation. As I always get the first 1 or 2 upgrades asap to get out of underdevelopment cost I was thinking on when to get additional upgrades for the planets. Also maybe someone can write when to research additional population capacity for terran/other planets in the civic tree.

It depends on how many you have.  Because of the way I play, I tend to get them after I finish expanding.  By this time, I have multiple terran/desert planets.  This way, you get more bang for your buck.. The research also allows for  a quicker return time since they already are at their normal population cap.  This adds onto it rather than researching at the beginning and then allowing it to slowly grow.

When it comes to quick-start, it's perfectly viable to do this for terran planets at the beginning of the game.  Desert planets are a little more iffy, since you don't start with one and even if you do colonize one it will take some time for its population to build up.  In general, you should have at least one planet with maxed out population levels before you pursue these upgrades.

Ice and volcanic population upgrades have an even higher standard before you should consider them.  These upgrades are more expensive and the planets themselves support less population (a 10% bonus on 70 population is just 7 pop.  A 10% bonus on 280 population is 28 pop).  It's extremely rare that you'll get enough planets of the right type with high enough population and loyalty for this upgrade to actually be worth your time.  I've only ONCE seen the ice population tech be worthwhile, and I've never seen the volcanic one be worthwhile.

if i find myself wanting more credits, i buy these upgrades for my terran/desert planets.

If I find myself wanting credits, I tend to spam trade ports...  Some argue against it, but within a couple minutes I have such a huge influx of cash that I don't end up using it all...  Though generally I use it to make some monster fleet...

The time to go trade totally depends on your situation.  I had a game the other day where I had a chain of asteroids, but otherwise all I had were ice and volcanics.  The result was I had no way to produce good tax income, but I had plenty of space for a trade chain.  Alternately, I had another setup recently where my surrounding area was pocked with neutrals that would break any attempts to trade chain, and I had a desert and a second terran nearby.  I just bought population upgrades on those planets and kept expanding.  It all depends on your situation.

Trade is a bit expensive to set up.  If you can get away with it, that's great, but usually you can't.