The Meaning of Life, the Universe, and Everything.
So tape might be a bit too glamorous of a term, but it gets the point across. An item that essentially counts how many blocks you've walked away from the block you right-click on.
This would save you having to parse through the F3 menu, which newer players might not even be aware of, placing torches at intervals or straight up having to go mad from counting hundreds of blocks (speaking of which, has anyone else ever noticed ttheir vision being a bit wavy after doing that?).
I can't say it's a perfect solution, but an idea I'm thinking of is an item/block resembling a tripwire hook. When you place the first one, a zero appears floating above it, a bit how you would see a pet or a player name when mousing over it. You can then use a second one, right click on the first one and place it wherever you want to measure to, at which point it would display the distance between them. Linking them up would allow you to measure perimeters, where as only pairing them would be good for measuring straight distances.
It does still pose the issue of trial and error for finding the right distance you have in mind when building something, which I'm afraid cannot be done without adding some form of live distance counter on the UI, whether it be down in the hotbar, by the cursor or somewhere else on the screen when holding the item.
Maybe there is a much simpler solution and I'm not seeing the forest for the trees, but I remember when the ruler was added in Terraria and how much of a convenience it became for building. Of course we have another dimension to work with, which complicates things, but I'm sure it'd be a welcome addition by many. Especially if planting these markers would mean that the more forgetful of us don't have to keep re-measuring every time we come back to a project.