GEN1288 You should know too, that the giving of one tenth to charity is the practice for the average person.  Whoever wishes to act generously, however, should set apart one fifth of his possessions.   Chazal have found support for this idea in [this verse]: “And of all that You give me, a tenth – a tenth will I give…” – twice one tenth equaling one fifth.   Here too, I believe, one is first to give a fifth of his net worth and then one fifth of all his subsequent earnings.   In distributing the funds, one should divide them into two: one tenth going to students of the Torah (corresponding to the first tithe given to the Priests and Levites to devote themselves to the study of the Torah, as Scripture has stated), and the second tenth for mitzvah purposes.   So I have learned from the Shittah Mekibetzeth (Kethubot 50a) quoting Harav Hameili.   AHAVCH 165