If the spiritual equivalent of tithing does not cover the provision
of the monetary requirements of the work, what principle in the Old
Testament does?

Deut 25:4 You shall not muzzle an ox while it treads out the grain.

Notice;


    1Tim   5:18  For the Scripture says, "You shall not muzzle
                 an ox while it treads out the grain", and "The
                 laborer is worthy of his wages."

    1Cor   9:9   For it is written in the law of Moses, "You shall
                 not muzzle an ox while it treads out the grain".
                 Is it oxen God is concerned about?
            :10  Or does He say it altogether for our sakes? For
                 our sakes, no doubt, this is written, that he who
                 plows should plow in hope, and he who threshes in
                 hope should be partaker of his hope.
            :11  If we have sown spiritual things for you, is it a
                 great thing if we reap your material things?
            
            :14  Even so the Lord has commanded that those who preach
                 the gospel should live from the gospel.



This principle does not involve a set percentage!



The person who chooses to give has plenty of freedom to consider their
financial circumstances. There is also no implied need to donate if one
is not benefiting spiritually from the labour of a minister or ministry.


Quite clearly however, if one is gaining benefit from the study of
another there is an obligation to return this favour through the
provision of material (or other) assistance.