The New Testament never specified a percentage.Times are a changin'. Next thing you know someone is going to say tithing should be based on net income instead of gross.
Traditionally The Church (Catholic/Orthodox) supported itself. The clergy, being educated, was tasked with making and selling things that were beyond the abilities of common people. Items like perfumes, beeswax candles, incense, soap, beer, wine and liquors, basically Chem 101. Many liquor brands today are still made by Catholic Monks such as Chartreuse and Benedictine. There are also the famous Trappist beers made by monks across Europe. Also IIRC the Catholic church also owns a large share of Beretta, but the Vatican Armory prefers SIG