How do we know what to set carb, protien and fat % at?

For most people, the MFP default percents are a reasonable place to start, as long as they don't cut calories silly-low for very aggressive weight loss for their current size. (There's no way to get adequate nutrition on too few calories.)

Those defaults would be:

50% carbs
30% fats
20% protein

I assume you must have a reason for resetting them from defaults?

I prefer to set my macros in grams. I have premium so I can do that directly now. When I had free MFP, I set the percents so base calories would come close to my gram goals, but ate to the gram goals in my head

MFP's blog now has a more nuanced macro calculator, too. It's here:

https://blog.myfitnesspal.com/free-macro-calculator/

As another option, there's a research based protein calculator and guide here (from a site with a decent reputation, that doesn't even sell protein supplements ):

https://examine.com/guides/protein-intake/

Note that the guide - at least last time I read it - says it's OK for someone very overweight to use a lower weight, maybe a sensible goal weight, in the calculator. We don't need bunches of extra protein to maintain our fat mass!

As a rule of thumb, I shoot for a minimum of 0.35-0.45g of fats per pound of bodyweight, and am OK if I exceed that. (Men might get away with a little less?) If bodyweight is currently very high, it could be OK to use a weight more in the normal BMI range to get a fats estimate, to, I think.

Since carbs - unlike protein and fats - don't contain things our bodies can't manufacture on their own out of other intake, I literally don't care where my carbs fall. As long as I meet or exceed my protein and fats minimums, and calories land where I need them, I'm satisfied. I also don't worry if I have a very rare day where I'm off from my goals, as long as the overwhelming majority of days come out where I want them.

Some people must manage carbs carefully, such as diabetics or those who are insulin resistant. I'm not one of those, plus have normal blood pressure and blood lipids since weight loss, so I don't worry about it. Most days now (in maintenance) I eat 225g or so of carbs, almost entirely from fruits, veggies, and no-sugar-added dairy. I'm routinely well within the WHO recommendation of no more than 10% of calories from added sugar, too.

But that's just me, with my size, activity level, age, health history. Not everyone is me! YMMV.

Best wishes!