Paul wrote by The Holy Spirit in 2 Corinthians chapter 9:
6 Here’s my point. A stingy sower will reap a meager harvest, but the one who sows from a generous spirit will reap an abundant harvest. 7 Let giving flow from your heart, not from a sense of religious duty. Let it spring up freely from the joy of giving—all because God Loves hilarious generosity! 8 Yes, God is more than ready to overwhelm you with every form of grace, so that you will have more than enough of everything—every moment and in every way. HE will make you overflow with abundance in every good thing you do. 9 Just as the Scriptures say about the one who trusts IN HIM:
Because he has sown extravagantly and given to the poor,
his kindness and generous deeds will never be forgotten.
10 This generous God who supplies abundant seed for the farmer, which becomes bread for our meals, is even more extravagant toward you. First HE supplies every need, plus more. Then HE multiplies the seed as you sow it, so that the harvest of your generosity will grow. 11 You will be abundantly enriched in every way as you give generously on every occasion, for when we take your gifts to those in need, it causes many to give thanks to God.
12 The priestly ministry you are providing through your offering not only supplies what is lacking for God’s people, it inspires an outpouring of praises and thanksgiving to God Himself. 13 For as your extremely generous offering meets the approval of those in Jerusalem, it will cause them to give glory to God—all because of your loyal support and allegiance to the gospel of Christ, as well as your generous-hearted partnership with them toward those in need. 14 Because of this extraordinary grace, which God has lavished on you, they will affectionately remember you in their prayers. 15 Praise God for HIS astonishing gift, which is far too great for words!
My wife and I can attest to the above passage in our lives as we have been generous givers to those in need. We choose to determine our level of generous giving by what The Holy Spirit instructs rather than using some formula.