My reasoning is that, if you spent $4.50 on lunch, you might as well tip $2, because it’s still nothing, really. (If $2 makes a difference to you, you should be eating lunch at home or packing a bag.) On the other hand, if you’re spending $100, you might as well tip $30, because you’re already putting it on the credit card, even though you know better.
I do tip for take out but 10-15% vs. 15-20% for eat in. My kids have never had to wait tables but my nieces and other friends’ kids have and it is hard work.