"Text copied from Web page/s must be taken inside the quote tags linked to the source/s and
total not more than X* percentages of the whole post content."
*-X is subject to discuss.
Regarding X, I would suggest 30%.
Personally, I like/support the initial part in your suggestion but AFAIK, there isn't an easy way of measuring "X"
[both internally and externally, unless I'm missing something].