(in which I dribble on about something I probably don’t understand)

…the thing is, you can’t prove anything without checks and measures, and establishing level of acceptability…

You can’t prove a building won’t fall down. You should be able to prove that it can endure certain forces for certain durations within certain thresholds of acceptability.

Similarly, you can’t prove a logo design will improve business, but you should be able to establish checks and measures and thresholds of acceptability, and prove that it has performed as required by those checks and measures.

You really need to know what needs measurement and checking, though. You need to focus on ensuring that you understand what needs to be checked and measured, and what is acceptable for each one.