No problem. I've definitely seen a lot of baking articles that somehow try and make this simple concept unbelievably convoluted.

The only other thing to know is that 1 mL of water = 1 gram of water. Which means 170g of water == 170 mL of water (At STP... blah blah blah. It's not super important to hit exactly 70% you can hit 75% or 65% and you'll be fine. It's close enough to true).

Nope. You aren't measuring the percentage of liquid in a dough. You are measuring the percentage of liquid relative to the mass of flour. That's why you can have 100% or higher hydration doughs.

In actual math, you are correct, but these are baker percentages where flour is always 100% and all other ingredients are relative to the flour.

So a recipe would look like this:

80% White Flour 20% Whole Grain 75% Water 15% Sour Dough Starter 2% Salt

Makes it really simple to scale recipes, you decide how much flour to use, for example 500g it becomes

400g White Flour 100g Whole Grain 375g Water 75g Sour Dough 10g Salt

Pro tip (really more of an amateur tip): Flour is a natural product that varies widely between different regions and there can be large differences in how much water they can hold and how much protein (gluten) it has. Hold back 10-15% of the water at first and only add it bit by bit when the dough feels dense and you think it (and you) can handle it. My biggest beginner mistakes were definetly trying high hydration doughs without the know how of how to handle such doughs and how to tell whether or not the flour could actually hold on to that much water. 65% Hydration can make also make a dope loaf that's much easier to handle