This is actually a very complex question! Part of the answer is that it depends both on the hydration level of your starter and its strength/maturity/ability to rise. The other part of the answer is actually another multi-part question - how much time do you have to devote to proofing and how sour do you want the finished result to be?
In your first example, you listed a recipe with 500g flour and 300g water, which is 60% hydration (amount of water divided by amount of flour). If your starter is also 60% hydration, then no matter what amount you add, it will not change the hydration level of the recipe.
However, if your starter is 100% hydration, it could change the hydration level of the recipe fairly significantly depending on how much you use. Say you add 150g of 100% starter, which is 75g flour and 75g water. Your recipe now has 575g flour and 375g water, or 65% hydration. That 5% difference in hydration is enough to change the texture of your finished product (not necessarily for the worse, but it will be different from what the recipe intends).
An easy way around this is to subtract your starter from the amounts of flour and water in the recipe. If you want to use 150g of starter at 100% hydration, your recipe would convert to 425g flour, 225g water and 150g starter (all other ingredients would remain the same).
As for the actual amount of starter, that is a matter of both desired speed and flavor, and depends upon the behavior of your starter. I store my starter in the fridge and only wake it up when I want to use it. I give it a day to wake up but it still tends to be a bit more sluggish than a starter that is store at room temp. Therefore I usually use more starter than most books would suggest - I often see recipes calling for 10% starter (so 50g starter in a recipe calling for 500g flour), but I usually use closer to 20%.
Then there is the question of how fast you want things to happen. Starter is slower than commercial yeast by nature, but if you want things to move faster, use more starter and/or proof the dough at a warmer temperature. You can also add some commercial yeast to the mix - it will provide a reliable and fast rise while the starter just adds flavor (although IMO this technique does not produce nearly as good a flavor as a starter-only rise).
Knowing how your starter will behave takes some trial and error. I generally only use starter if I know I have plenty of time to let the dough proof before I want my bread. Usually an overnight bulk rise followed by 4-6 hours after shaping works for me, but sometimes I put doughs in the fridge to allow them to slow ferment for a few days.
Final question is flavor. How sour do you want the finished product? My starter produces more sour flavor with warmer proofing. Cold proofing gives me less sour but more complex flavor. Again, as you get to know your starter you will be able to judge better what flavor it will give you under what conditions.
As to how to judge whether your starter is strong enough to raise bread on its own, if it reliably doubles in size after feeding at room temperature within 6-8 hours, it is plenty strong enough! As I mentioned, mine lives in the fridge so it sometimes doesn’t pass this test, but it will still raise a loaf if I give it enough time. I try to make sure mine is active but hungry when I mix it into a dough, by which I mean it has been removed from the fridge and fed a couple of times over the preceding day or two, but is ready for its next feeding. The flour and water in your recipe effectively ARE its next feeding! The float test mentioned earlier may or may not apply depending on the hydration of your starter and whether or not you de-gassed it before testing.
Hopefully this helps! If you let me know what hydration your starter is, I would be happy to help you figure out how to convert your original recipe for starter only. 
