Why does there have to be a clear line to make the marginal argument valid? For individuals, I will state: spending a marginal dollar on charity is more moral

Why does there have to be a clear line to make the marginal argument valid? For individuals, I will state: spending a marginal dollar on charity is more moral than spending a marginal dollar on luxuries. I don't have a clear answer to where the line lies, even for myself. I try to keep giving until the marginal luxuries which I give up have greater utility than the utility I gain from donations. Admittedly, this is not exact or imprecise. My giving is strongly affected by my mental health (one of the best ways I've found to get out of a depressive spiral is to go on a donation spree). However, I don't think this lack of precision undermines the notion that charitable giving is more moral than luxury spending.