Dividing the board length by the piece length gives the wrong answer, and it gives it in the expensive direction. Every cut removes a slice of wood as sawdust, and on a board with several cuts that slice adds up to a piece you thought you had.
What kerf actually is
The kerf is the width of the slot the blade cuts. It is slightly wider than the blade body because the teeth are set outward, and it is entirely lost — that wood becomes dust.
A standard full-kerf table saw blade cuts about an eighth of an inch. Thin-kerf blades cut about three thirty-seconds, which is why they need less power. A bandsaw cuts around a thirty-second, and that is the real reason resawing is done on a bandsaw rather than a table saw.
An eighth of an inch sounds trivial. Eleven cuts in an eight-foot board is nearly an inch and a half of wood gone.
Where the cut goes matters as much as how wide it is
The blade has to be on the waste side of the line, every time. If you cut down the middle of a pencil line, the piece ends up short by half a kerf plus half a pencil line, which is close to a sixteenth.
On one piece a sixteenth does not matter. On a set of shelves that all have to be the same, it means every shelf is a different length depending on which side of the line you happened to favour.
The practical answer is a stop block. Set it once, cut every piece against it, and they come out identical regardless of where the line is.
Cut the longest pieces first
Working from longest to shortest means the offcuts stay usable. Cut the short pieces first and you end up with a pile of lengths too short for anything you still need.
The calculator assumes all pieces are the same length, which is the common case for legs, shelves and rails. For a mixed list, run it once per length starting with the longest and feed the leftovers into the next run.
That is also the order a cut-list optimiser works in, and for a handful of parts you can do it faster on paper than in software.
Buy longer than the arithmetic says
Boards are rarely usable to the very end. There is usually a check, a split, or a stapled tag in the last few inches, and the factory ends are not square.
Trimming an inch off each end before starting is normal, and that has to come off the usable stock length before this calculation, not after.
If the result comes out to exactly the number of boards with almost nothing left over, buy one more. The alternative is a second trip for one board, at which point the lot number and sometimes the colour no longer match.
Rough length and finished length
Pieces get cut twice: rough, oversize, so they can be milled flat and square, and then to finished length after milling.
Rough length is usually an inch or two longer than finished. That allowance is what lets you cut off the snipe from a planer or the burn mark from a stopped cut.
Enter rough lengths here if you are milling your own stock. Entering finished lengths and then milling will leave you short on the last piece.