A replacement crate slat can come up short even when the measurement was right. The saw removes a strip of wood, called the kerf, and that strip has to come from somewhere. If you centre the cut on your finished-length line, part of the wood you meant to keep becomes sawdust.
Start with one sound, square end as your reference. Measure the needed length, make a fine pencil mark and carry it across the face with a square. Put a small cross on the offcut side, away from the line. That cross is a quick reminder of which piece you are willing to lose when the board changes position on the bench.
For a handsaw cut, secure the slat on a suitable bench hook or with clamps, keeping the cut clear of the bench and metal hardware. Keep your free hand out of the saw path and support the work so it cannot shift. Start gently on the marked waste side, leaving the length line visible on the piece you are keeping.
If you are unsure how closely you can follow the line, practise on a similar scrap first. Leave a little extra length on the replacement and work down to the mark with a suitable sanding block or plane. Check the end with a square as you go; a long point on an angled cut can make the part seem to fit until you try to close the joint.
Test the slat in its actual position before drilling or gluing. It should fit without forcing the surrounding box out of shape. If you need several matching pieces, finish and check the first one before using it as a reference. Do not mark a whole batch end-to-end without allowing for the wood each saw cut removes.
Practical takeaway: make the finished-length line, cross the waste side and keep the saw on that side. A pencil cross will not guide the blade for you, but it can prevent a perfectly measured repair part from becoming a slightly too-short offcut.
Hand ToolsMeasuringCrate Repairs