Good question. IMO the effect would most closely resemble a refundable lump-sum tax rebate, except that it would happen every month, not just once. Traditional theory (permanent income hypothesis) says making in permanent would have a larger stimulus than a one time rebate. So I would start from there. But remember, you have to deduct any programs that are replaced by the UBI, e.g. TANF, mortgage interest deduction, etc.