The Level Line Every Cabinet Hangs From
Find the high point before anything else
The first tool out of the van is not a drill. It is whatever establishes level across the whole room: a rotary or cross line laser set in a corner, or a 6 foot straightedge with a good vial walked along the base of every wall. The job is to find the single highest point of the finished floor along any wall that will carry a cabinet.
Floors move. A slab poured on grade will typically read 1/4 to 1/2 inch of fall across a 12 foot kitchen. Framed floors over a crawlspace do worse, and in older valley housing stock around Yakima a joist that has taken on a set will give you 5/8 of an inch of dip in the middle of a run. It does not matter. What matters is that one spot is higher than everything else, and every base cabinet in the room has to sit at or above the height that spot dictates. You cannot shim a floor down.
Snapping the reference
From the high point, measure up the finished base cabinet height, normally 34 1/2 inches for a 4 1/2 inch toe kick plus a 30 inch box, and mark. Transfer that mark around the room with the laser, not with a level walked along the wall, because a 4 foot level walked in three bites accumulates its own error and will hand you a line that climbs. Snap a chalk line at that height. Then measure up again from the same reference to the bottom of the wall cabinets, usually 54 inches off the floor high point, which leaves the standard 19 1/2 inch backsplash gap once counters go on, and snap that too.
Now the room has an origin. Every measurement after this is taken from a line, never from the floor, never from a neighbouring box, never from the top of a base cabinet that has not been shimmed yet.
Why the first upper box decides the rest
Uppers usually get hung first, because it is easier to work over an empty floor than to reach across finished base cabinets. The first box goes in a corner, sitting on a temporary ledger screwed to the studs along the upper chalk line. That ledger carries the weight while it is being fastened, which is the whole reason experienced crews use one: a box held by a knee and a shoulder gets fastened wherever it happens to be, not where it belongs.
Fasten through the hanging rail into framing with 2 1/2 inch cabinet screws, two per stud, and check plumb on the face before the second screw goes in. That first box is now a physical datum. The next cabinet is clamped face to face with it and shares its plane, the third shares the second, and so on. Any error in box one does not stay in box one. It transfers down the run and multiplies, because each following cabinet is aligned to its neighbour rather than to the line. A 1/16 inch lean at the corner reads as a visible taper by the fifth door.
The two shortcuts that ruin a run
Working off the floor is the common one. A crew measures 54 inches up at each stud rather than transferring one line, so the upper cabinets follow the dips in the floor, and the gap between the counter and the upper wanders by half an inch across the room. Nobody sees it until the tile goes on and the last row has to be ripped tapered.
Working off the countertop is worse and happens on remodels where old counters are staying. The counter itself was scribed to a wall that has since been patched or rebuilt, so it is not level, and it is not straight either. Referencing new uppers off it inherits every error the previous installer made and adds new ones on top.
There is one more: hanging the whole set and only then discovering the appliance opening has drifted. A 30 inch slide in range wants a 30 1/8 inch opening, and there is no adjustment left once the run has been screwed to the wall on both sides of it. Mark that opening on the reference line before the first box goes up, and build outward from it in both directions.