A4 and US Letter are close in size, but they have different proportions. A4 measures 210 × 297 mm. US Letter measures 8.5 × 11 inches, or 215.9 × 279.4 mm. Letter is wider and shorter, so matching one edge does not make the other edge match.
Fit the entire page
With no margins, the A4-to-Letter width ratio is 215.9 ÷ 210, while the height ratio is 279.4 ÷ 297. To keep the whole page, use the smaller ratio: approximately 94.07%. The height now fits, and the scaled page is about 197.56 mm wide. The unused width becomes side space if you center the page.
Account for real margins
If your printer needs a 5 mm margin on each edge of Letter paper, the available area is 205.9 × 269.4 mm. The same A4 page now fits at approximately 90.71%. Enter your printer's actual margin in the tool instead of assuming that the entire physical sheet is printable.
The margin setting in Swetter is symmetric: it reserves the same distance on all four edges. If your printer has different minimum margins, using the largest as the tool's margin is a conservative starting point. The calculation does not inspect your PDF's existing white space.
Fill has a different purpose
Fill uses the larger ratio. Without margins, A4 enlarged to Letter width is about 102.81% of the original. The resulting height exceeds the available height, so part of the page must be cropped. That may be useful for artwork, but is risky for documents containing text near the edges.
Apply the result once
Set the correct destination paper in the printer dialog. If you enter Swetter's percentage as a custom scale, avoid adding another Fit adjustment afterwards. Software labels vary, so inspect the print preview. Adobe documents the difference between Fit, Actual size and Custom Scale in its page sizing guide.
For patterns, rulers or anything whose dimensions must remain exact, do not silently rescale the page. Use the intended paper size, print at 100%, and measure a known reference length before using the output.