How do you tip at restaurants?

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In the US, when you dine out at a restaurant, there is usually a suggested tip amount for you to leave displayed on the receipt. Usually it's 15%, 18% or 20%, though sometimes another suggested amount may be displayed, like 25% for example. I've noticed that sometimes in the receipt that lists everything that was ordered you have these suggested amounts and they are based off the subtotal (w/o tax), but then when you get the receipt for your swiped card you get the suggested tip amounts again but they are now for the total amount (with tax). So, which amount should you leave a tip on - the subtotal (w/o tax) or the total (with tax)? I feel like we should tip on the subtotal because why should we pay a tip on tax? It just makes no sense. However, enough times when they swipe your card and bring you the receipt to sign, the other receipt that listed all the items and had the subtotal is no longer there, so you can't know what the subtotal was and hence can't leave a tip based on that amount. And when it comes to tipping at restaurants, I feel like you should leave as much or as little tip as you want. It shouldn't be necessarily 15%, 18% or 20%. It should be whatever you feel like giving or can afford to give. Any amount should be appreciated. Though I am sure there are waiters that would be upset if the tip you leave is too little and would complain about it, maybe even confront you if that happens. And there are times when a set tip is charged automatically, which they call a service charge or gratuity. I think that is wrong to do. Who said that the customer wants to give that much tip or can afford to give that much tip? Tipping should be decided by the customer and not the restaurant and no matter how large the dining party is. Also, automatically charging a service charge and then also asking for tip in the end is wrong. To me, service charge and tip are for the same thing - the service. So, why should we have to pay that twice? How do you tip at restaurants? On subtotal? Or total? And how much do you usually leave?