You got the gist of it. Although....
It depends a little on your customer. I vape around 50w but own a 100w mod and a 200w mod. I have them because they take two batteries, so last longer. So selling a 20w tank with a 200w mod MAY be overkill but MAY be suitable, so long as you can gage that from the customer and that they understand the right wattage range for their tank.
I will wager that every single person who joined this forum has sounded dopey on occasion, I know I do it on a regular basis, most of us joined to learn, so don't feel worried about asking questions. We would ALL prefer our b&m staff to know what they are talking about, so keep on asking!