How do I know if my ledger is actually level enough for the joists?

Pressure-treated lumber is notorious for being inconsistent right off the truck. Since you're working with a frame that's only four inches off the ground, that 'hair' of a difference is likely just a slight crown in the joist or the wood swelling unevenly from the treatment chemicals. You shouldn't expect furniture-grade precision with wet PT lumber. If the bubble is still touching the lines on your level, you're usually within the acceptable margin for a residential deck. But you need to be careful with the foundation side of things. If you're resting that frame on a concrete slab or using pavers with scrap wood shims, those shims are going to be your biggest headache later. Wood-on-wood contact near the ground traps moisture, and those scraps will rot or compress long before the main joists do. If you need to level things up this early, use **composite shims** or plastic spacers rather than wood scraps. They won't compress over time, wich keeps your deck from developing that roller coaster look once the weight of the decking and furniture is added.

Since you haven't nailed the hangers off completely, now is the time to check for crowns. Flip your joists so the high side—the 'hump'—is facing up. When you screw down your decking boards later, their weight actually helps pull that crown down and flattens the surface. If you have one joist that is significantly higher than the one next to it, don't be afraid to take a power plane to the top edge to knock it down. It's much easier to shave a quarter-inch off a joist now than it is to fix a trip hazard later. Don't worry about it being perfectly 'level' in the sense of a zero-degree flat plane, either. You actually want a slight slope—about an eighth of an inch per foot—away from the house siding. This prevents water from pooling against your threshold or the ledger board. If your 'off by a hair' measurement actually favors a slight slope away from the house, you've accidentally done it exactly right. Just make sure your supports are solid; if you're just sitting on pavers without footings, that ground is going to move during the first big rain anyway.

Don't sweat the small stuff too much. PT wood's gonna move on ya anyway as it dries out. If you're really worried about that 'roller coaster' look, grab a chalk line once all the joists are in. Snap it across the top and you'll see real quick which ones are sitting high. I usually just hit the high spots with a belt sander or a plane like the other guy said. It's way faster than messing with hangers. Also, check your level's accuracy by flipping it 180 degrees on the same spot. Sometimes it's the tool that's off, not your work.

If you're allready losing sleep over the bubble, you need to ditch that basic level and grab a Stabila 96M-2. Most of those cheap box store levels have a margin of error that makes you second-guess everythign near the house siding. A Stabila is dead accurate and won't give you ghost readings when you flip it.

Also, look into the Camo Marksman Pro for when you start laying boards. It uses side-entry screws that pull the decking tight to the joists and keeps your gaps perfectly uniform. It's the best way to mask those tiny crowning issues you're worried about.

Back in the day, we didn't have these fancy composite shims or laser levels. We just used a water level—piece of clear hose and some dyed water—and it never lied. If your bubble is jumping, check your ledger attachment. If you didn't use trough-bolts or structural screws like TimberLoks and just relied on old-fashioned lags, you might have some sag starting alredy. Also, don't just trust the hangers; I always toenail the joists into the ledger first to lock 'em in before the hanger ever touches a nail. Keeps things from shifting while you're banging away.

Quick update—you guys were right about the PT wood being inconsistent. I checked for crowns like the first reply suggested and found a few joists were just sitting high. I used a chalk line to mark the high spots and planed them down. It’s looking much better now. I guess I was just overthinking the bubble! Thanks for the peace of mind.