Learn English – Can you help me with/about/on that

I'll switch the "goal" to something a little more specific...

If you say "I am trying to eat more fresh fruit", the normal implication is you're making that effort right now. It doesn't say much about how long you've been trying, or how successfully. So most likely there's also the implied meaning that you haven't reached your goal of eating sufficient fresh fruit yet.

If you say "I try to eat more fresh fruit", the normal implication is that by habit, for some significant amount of time, you've been eating more fresh fruit than you would have done if you hadn't bothered to make any effort in that direction. So must likely the implied meaning is you do in fact eat enough fresh fruit, but you're aware (and are reminding someone else) that it requires some effort for you to achieve this.

OP's "I try to calculate the minimum angle..." would therefore be an unlikely thing to say, because you wouldn't normally be in the habit of doing this. That's not to say it's an invalid construction - it's perfectly grammatical. But you'd probably only use it in some contrived "narrative" context chronologically and sequentially describing each thing that you do, as you do it.

I'm trying to think of a context where there is a difference, and it's very hard to do that. What makes this tricky is that the two are pretty much interchangeable in many contexts:

When I think about/think of my mother, who passed away three years ago, I get very sad.

but there are other occasions when one would be slightly (or clearly) favored over the other:

I get excited when I think about football season getting underway.

I can think of 10 reasons why you shouldn't text and drive.

And then there's this gem:

I can't think of it right now, but maybe I will, if I think about it long enough.

If I had to summarize, I'd say that think of seems to mean recalling something specific, while think about seems to mean considering some subject in a more vague or general way for some length of time. The two are sometimes interchangable because there are many situations where you can't think about something without also thinking of it, such as when you are fondly remembering your deceased mother.

^{This is a great question; I hope others weigh in. I think I might be getting close to something here, but I'm not convinced I've nailed it.}