You can give an accurate enough estimate. Yes, you may have to make some assumptions on protocol and server. And yes, you will be a few percent off. But so what? It is much more informative to the average consumer than transmission rate. In the first place, transmission rate is also a meaningless metric without information on overprovisioning, latencies, etc. Except instead of an understandable but slightly inaccurate download speed estimate, you have one engineering parameter that is unhelpful to an engineer anyway without more information as well as not understandable to the average consumer.
PS: I am not saying I can't include engineering parameters, but it shouldn't be the main marketing number.