This is my approach too. I only ever measure the liquid - water or sometimes water with a bit of milk. And measure is a strong word - I just fill 2 cup measuring cup I have because that gives me enough for two large loaves. It doesn't seem to matter much how much starter you put into the batch, as long as it's 'enough'. I usually almost-empty my partly full quart jar of starter. A glug of oil or a pat of butter, a small palm full of salt. As long as you add flour gradually until you get the desired consistency, you're fine. Maybe you get longer/shorter rising times, but I usually do cool and slow rising for hours and hours so I don't really notice much of a difference there.