I use a "cheat sheet" for sencha. The yuzamashi (water cooler) I use 90% of the time has iron spots in the glaze. I know where 3, 4, 5, 6, 7 ounce spots are on the inside of the yuzamashi.
I determine how much tea liquid I want to brew before picking the appropriate pot, then use the yuzamashi spots for the right amount of water.
For gyokuro, I more often use the volume of 1-2 small gyokuro cups to determine/measure how much liquid I will use.
For other teas, I would say the pot size more determines how much water I will use as I approach these teas much more casually.
Began the TD with Miyabi with the Mrs. I predetermined I needed 5 ounces of water and filled the yuzamashi to the 5 ounce spot.