This is where a proportion scale works well. If Onebyone has figured correctly, and the enlargement is 150%, make a copy on your printer at 150% and add the appropriate 1/4" per segment. With a home copier it will take several sections to make a complete 20" block, but a commercial printer or copy center could make one with no trouble. It would be better to have a hard copy to have duplicated, but a quilt block may work if it is square.
Good luck!