If a shop person looks at you and says, "You are about..." find another shop. Rapidly.

All top bike shops have a system to fit customers and their system will also tell you which manufacturer's frame works best for you. Top shops will use the top tube as the measuring standard, then will also be able to tell you what stem length you need (if the stock stem is not right...like you have a long torso, you'll need a longer stem on what's a shorter bike due to your leg length being shorter for your height, if that was the case).

The old "stick up the crotch" system is also inadequate. It does not take body dimensions (leg length to torso) into consideration.

You should get two things measured...your leg length (NOT YOUR INSEAM) and your height. Those two numbers can be used to give us a ratio and then that number can be used to determine the proper top tube length. From there, if you have a need, a longer or shorter stem may be necessary.