The principal, the example of giving is set prior to the law, when Abram gave a tenth of all to Melchizedek.
The gift of Abraham was a one-time gift of the spoils of war. It is never repeated again in scripture regarding Abraham nor is there any evidence that Abraham “tithed” regularly.
After this you get into the promised land and the Levitical priesthood under the laws of Moses. Certainly not applicable to Christians today because we don’t observe these laws - nor was it the 10% monthly, nor do we celebrate, nor do pastors preach it correctly.
The only mention of tithing in the New Testament were directed at hypocrisy in the Temple. Jesus lived under the law - but he removed us from the law.
We should give out of love as directed by the Holy Spirit. The reality that we have “Christian” churches bogged down in 10% pre-tax vs 10% post-tax argument saddens me.
I think you miss the point: if 10% was good enough for Abraham, 20% seems above and beyond the call of duty.