What you are doing here, Craig, is taking a perfectly legitimate way that measurement works in every area of science, and twisting it to portray it as if it were some sort of circular reasoning when it is not.
Of course you need to know a ball park figure before you measure something! You simply need to know this in order to select the right tool for the job. If I want to measure the size of my desk, I need to know that it is somewhere in the region of 2 metres by 1 metre by 1 metre, so that I know to use a tape measure rather than an electron microscope, a micrometer, a GPS device in my car, or the Hubble Space Telescope. I then use the tape measure to get an exact size of 1.5 metres by 0.75 metres with a height of 0.72 metres. It’s nothing to do with getting the result that you want to get. It’s about starting off with a highly uncertain estimate (to within a factor of two or so) and narrowing it down (to within about one part in a thousand).
By YEC reasoning, the fact that I start off by making a finger-in-the-air estimate of 2 metres by 1 metre by 1 metre would mean that using a tape measure to check that estimate would be “cooking my conclusions” and that I should just accept the alternative narrative that my desk could be no bigger than the width of a human hair.