That’s not true. I’ve heard accounts of early servers who only made money off of tips and were expected to pay the restaurant a portion of their tips for the privilege of serving there. In fact the only way to get service was to tip them beforehand and how much you tipped determined the level of service you received. This tip for good service is just a myth that is an excuse to avoid tipping.
Try greater than 40%. A tip is for service provided directly to you. If the employer increases the servers wages by 20% then that’s every hour they work and there may not be any customers for some of the servers shift.