You are really overthinking this. When the server grabs lettuce or whatever you ask for and it’s too much just tell them before they toss it into the bowl. Most servers will take a portion of each ingredient and ask if it’s okay. Obviously, nobody can monitor how much people will eat. However, if you only take 50% of what you normally would pile on your plate, then there will be less waste. This is only a salad or whatever, not rocket science.