That's exactly what they figure. IF you go back and look at your past bids, you'll probably see that your costs per SF of wall area are pretty similar for similar jobs. For example, take 5 brick buildings with punched windows, and divide your total cost by the overall sf of the exterior, and I'll bet they're pretty close to each other. You can then look at the LF of sealant you had to apply relative to the SF of wall, and figure out that ratio.The areas in question are joints so that's a lineal figure whereas dampproofing type apps are SF. I guess they figure I can convert without plans???
Now when a GC calls with "how much to caulk XXX sf of wall", you can reply with "approximately $xx/sf, based on an average of YY lf of joint per 100 sf". Now when they send the actual plans over later, you can hit them for a change when the actual LF differs from what you quoted.
And yes, if they're calling you at the last minute and not sending plans with enough time to properly quote the job, they're either using you as a check number, or they're doing a conceptual estimate. In most cases, it's probably the former.