Mechanical Ruler

Builders, and nobody else. It lays a faint grid over the screen so you can count tiles without squinting, and it draws a crosshair through the row and column under your cursor, which is how you line a platform up dead level with something forty blocks away. The grid fades while you move and returns when you stop, so it is not sitting in your face while you walk around.

Informational accessories work from your inventory, so this can live in your bag or your Void Bag and function normally. Giving it an accessory slot buys you nothing but the modifier. A toggle sits next to the inventory so you can switch the overlay off without dropping the item, which means there is no cost to simply carrying it forever. Putting it in a social slot does turn it off, so keep it out of there.

If the overlay is too loud for you, the game's own tile grid option draws a much smaller version right around the cursor for free. Keep the ruler anyway. It goes into the Grand Design, which is the wiring tool you will want eventually, and re-buying it later is one more errand you did not need.