http://turbotax.intuit.com/ 

Does that help?

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.

Unless this info is seriously outdated, I think that each unit of population = 0.025 credits/sec, such that 40 population = 1 credit/sec (this is without allegiance, which acts as a simple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

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.

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 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...