A part that measures 99.5 when it should be 100 is a half percent error, and half a percent is the difference between a bearing that presses in and one that falls out. The correction is simple arithmetic; deciding where to apply it is the part that matters.
Scale the model or fix the machine
Scaling in the slicer is instant and applies to one model. It is the right answer when you need this part right now and the error is small.
Correcting steps per millimetre fixes the machine, so every print afterwards is right including models you did not design. It is the right answer for anything other than a one-off.
The arithmetic is the same ratio in both cases: nominal divided by measured. The calculator gives both so the choice is yours rather than the tool's.
Measure long, not short
A proportional error shows up in proportion. On a 20 mm cube, a half percent error is 0.1 mm — inside caliper noise and inside the variation from corner bulge.
On a 100 mm part it is 0.5 mm, which is unambiguous. Print the longest calibration object your bed allows and measure that.
Measure across flats, not corners, and not near the first layer. Elephant foot at the base makes the bottom read large on every part and has nothing to do with scale.
What is not a scale error
Holes come out undersized on almost every printer, and that is not a scale problem. It is a combination of the nozzle cutting a polygon inside a circle and the plastic pulling inward as it cools. A hole 0.3 mm undersized on a part that is dimensionally perfect is normal.
Outside dimensions coming out large while inside dimensions come out small is the classic signature. Scaling the model fixes one and makes the other worse.
The fix for holes is to design them oversize, or to use the hole compensation feature slicers now include — not to change scale.
Material shrinkage is a real term
Filament shrinks as it cools, and by different amounts: PLA very little, ABS and ASA noticeably, and nylon considerably. A large ABS part can finish nearly one percent smaller than its model.
That is why some slicers have a per-material shrinkage compensation separate from scale. Using it is better than changing steps per mm, because the machine is not wrong — the material is shrinking.
If your PLA prints are dimensionally perfect and your ABS prints are consistently small, do not touch the axis. Compensate per material.
How accurate is reasonable
A well-calibrated hobby printer holds about plus or minus 0.1 to 0.2 mm on dimensions up to 100 mm. Chasing better than that with steps per mm is chasing noise.
Consistency matters more than absolute accuracy for most work. A printer reliably 0.2 mm small is easy to design around; one that varies randomly by 0.2 mm is not.
Print the same test part three times before concluding anything. If the three differ from each other, the problem is mechanical and scale is not the answer.