Test with a 71 ft (21.6 m) wire. Expect SWR under 2:1 on 40, 20, 15, 10m with a good counterpoise.
Search for “balun construction handbook” filetype:pdf or visit archive.org/details/balun-construction to find scanned ARRL and RSGB handbooks. Building And Using Baluns And Ununs Pdf