The number we care about is how much currently going to heirs, which is 36 Trillion. US accounts for about 25% of global GDP, then (kinda maybe) that means 144 trillion globally, which would be 2.6 trillion a year. Perhaps larger if we also take from government pool since less goes to taxes if raha is a donation.
One could argue we care more about the amount currently going to charity. We need to position Raha such that members realize donating to charity is a more effective means of alleviating poverty today, and encourage them to do so by aligning ourselves with projects like Giving Pledge. Raha wants to take from the portion that would go to their kids, and to change people default behavior. One day, in a will think about “how much to charity?” and “how much to my kids?” but make the default of our redistribute economic operating system go towards supporting UBI.