Lets assume your bylaws make sense (or were intended to make sense) and that your quorum requirement, however poorly worded, is two-thirds of the voting membership and that the voting membership includes only dues-paying members. As you correctly observe, with 80 dues-paying members you'll need at least 54 of them to show up if you want to conduct any substantive business. Any business conducted in the provable absence of a quorum is null and void. The solution, of course, is to amend your bylaws to reduce the requirement (and, at the same time, tighten up the wording). The RONR default quorum is a majority (more than half) of the membership so that's as good a starting point as any. You can either specify a percentage (e.g. 25%) or a fixed number (e.g. 25). There are advantages and disadvantages to each. [by the way, bylaws that try to appear precise by repeating "two-thirds" as "2/3", as if there could be any confusion, seem more likely than not to be poorly written.]