Choose the finished page first
A5 is 148 × 210 mm. Half Letter is 5.5 × 8.5 in. Choose the actual finished dimensions rather than scaling a different page until it looks close.
Use a source sheet that fits
Choose A4 or US Letter as the source sheet, then a centered or two-up arrangement. The builder refuses a fit that would require scaling. Download the PDF and print at Actual size.