I need to cut 1/4” off the bottom of my door. really struggling and wondering if the teeth on my saw are too much - seems like 140? Could I use a 50 tooth? And how did I fix this? Can I move the level/guide 1/16 inch back and start again? I’m not a carpenter, so keep it in layman’s terms please.
I'm not surprised that tore away from the edge of the work-piece. Here are some ideas to get a better cut:
Get a fresh finish-cut saw blade. The blade shown in your picture is well-worn.
Put something beneath the door while cutting it to avoid tear-out. If you were using a table saw, the table itself would be helping, except this cut is so narrow it would depend on your table saw configuration. Since you're using a circular saw instead, get a piece of scrap plywood to set the cut area on top of. Set the saw blade depth so it cuts into the plywood slightly, but not all the way through it.
The advice is good in the other answers too. Your blade has a build up on it that will cause the blade to wander, typically from heating up and distorting, very difficult to cut a straight line with that happening. Do invest in another blade. I also use something on the face of the door, like thin cardboard, taped down, or tape the bottom of the saw so the metal base does not leave marks on the painted door. Another way to insure no tear out, is instead of marking with a pencil, carefully score the cut line and cut along that. With the new blade stay just a hair's width away from the score line, and when finished the cut, wrap a piece of medium to fine grit sandpaper around a block of wood and sand the corner only, holding the block at a 45 degree angle while sanding to ease the corner a little. This help keep the edge from chipping while resetting or from something getting caught under the door while opening and closing.
To answer the question about correcting the over cut, yes just shift the line the amount to need to incorporate the over cut into the re-cut.
The saw blade that would be good for all around use, and get the nice cut you need would be a carbide tipped blade between 24 and 40 teeth. I would determine which blade for you by what do you plan to use it most for. If it is general cutting or ripping the 24 tooth will do fine, even for cutting the door bottom. If you plan to do more with finished material, then the 40 tooth is a better option, although it will do general cutting just fine as well
For a smooth cut that doesn't rip wood pieces from your door surface you can't go wrong by buying a new saw blade. The more teeth the finer (less tear-out of wood) it cuts. You can get almost identical results from a new 10,18, 24 toothed saw blade. The best cuts are when it's new.
From looking at the first 2 photo's it looks like you deveated from the cut line. The way to amend this is to start cutting again where you left off and complete the removal of 1/4 inch.
If you find you still need to remove a hair more it will be difficult to keep the blade from deflecting from the line. You could use a plane or belt sander.
go slower, wear hearing protection it's going to be noisy, and you need to go slow, wear eye protection there will be sawdust flying in your face and you need to see what's happening, wear a dust mask (or just hold your breath)
My saw has about three tungsten carbide tipped teeth per inch and it works ok on doors. your saw appears to just have metal teeth, if the teeth aren't sharp it won't cut well.
I see the paint on your saw blade is all smudged that's from trying to cut curves. put the other end of the door against something solid so that you can push on your end without it moving.
Keep the saw straight and level. It's hard working with most of the foot of the saw hanging in space, if you can find a lower profile, wider, straight edge you can have most of the saw resting on the door because the clamps will be well away from the motor. I use the factory edge of a plywood offcut,
