To get a proper level, you have to start with a repeatable squareness. Nothing you’ll do will be consistent if you’re not starting from the same spot.
Your feet should all be pretty much the same height, you should enable the motors when both x rollers are the same distance from the ends and the y rollers are the same distance from the ends. You also need to check that the router bit is perpendicular to the work surface. Ryan made a printable part that attaches to the dw660 chuck and can check this, but you could also put a piece of bent wire in the router and slowly spin it to see if it’s always the same distance from the bed. (Does that make sense? The tip of the wire would make a >6" diameter circle when you spin it, and if it’s off perpendicular then some parts of the circle will be closer to the board).
After that, you really care that the bed is parallel to the gantry, not really level. The sure fire way is to surface the spoil board. You can also surface your workpiece (workpieces are rarely the same thickness everywhere). If you surfaced your spoil board, you can just run workpieces through a thickness planer. Also make sure you’re getting good clamping.
To set your Z, at the start, or after a tool change, you can use the 3d printer trick of putting a piece of paper over the spot you’re zeroing on. Jog the z down until the paper has some resistance, but isn’t stuck. That’s like a cheap feeler guage. This is the only way I’ve done tool changes, since I haven’t bothered with a probe. If you want to see if your workpiece is parallel to the gantry, you can do this in several places and look at what the Z value is at each place. This is a very manual way of doing mesh levelling :). If you measured 5 spots, one on each corner and one in the middle, you would have a very good idea of just how off it is. Then you can try things like resetting everything to find your repeatability or you can surface the board and try again. You’ll end up with a very scientific approach to this, I hope.
W.r.t. v carving, a sharper pointed bit will also be less sensitive to z errors. If you’re using a 90degree bit, then 1mm error will end up being 1.4mm wider. A sharper bit would be only 0.5mm.
Take the parts of all that that work for you. You can easily spend a whole day on this without cutting anything. I would enjoy that, but aome poeple might not :).
