This will result in failing sooner. Secondly, how much weight do you add. First, go into the weight room. Continue this until you can only do one rep. I'm guessing you were sharing a chuckle with us.
Warmup sets are the traditional method.
Too few warmups and you won't be able to exert maximum effort. Too many, and you'll be too tired. This is especially true for beginning and intermediate lifters attempting squats or deadlifts without experienced spotters to check their form and assist if the weight is too much. The 1RM is usually needed to calculate sets for use in a progressive program, in which case an estimate based on 5RM or 8RM, while possibly less accurate, is safer and easier to obtain. The first is based on estimates.