Slightly offtopic, this is about time studies.
You know, I must be in a nice spot. I came in to a company expect to have to things like PSE, I.E. observe covertly. But I have seen to have stumbled into a trick, dont know if it works all the time so YMMV.
Talk to the operators, something around this,
"You are timing not them, but the job that they are doing, and dont really care who is doing and that you will be mixing the operator times up anyways, so its totally anonymous. That you really worried about making it unreasonable, both for them and the company that employs them. Cant have it set too easy, because then the company will lose money and ultimately we will get a bad paycheck/lose it, and cant set it too high, because then its unfair for you and I know doing a bad job sucks. If you have any suggestions on improving job feel free to tell me and I will see what I can do."
After giving that speech (and asking their foreman/teamleader for permission) I seem to have gotten the floor on "my side." About half of the job improvement suggestions actually come from pretty good questions/and suggestions on the floor.
Ofcourse a lot of that comes from prompt feedback on their suggestions, both good and bad.
And, yes occasionally, at the beggining I was having to rate someone working at 80% efficiency (I havent told them I do that or other IE tricks), but after a while you kinda get accepted and get decent times. Some of the more motivated even go a bit faster for some reason, and I have to scale it back too. (I dont tell them that as well, though I do compliment either the operator or the team leader when I see it.)
Heh, now I get team leaders coming to me wanting to document everything that they do on the line.
Might be just my particular mix of people so your mileage may vary (we are non unionized). But I cant stress enough on getting people involved.
So for your case, I would alter it by saying you are looking for ways to make it, "fair." Take suggestions. Work with em a bit. Say what you think is reasonable, then query them if that is reasonable to them (maintain control of the conversation like that, and start with the "good" operators first). After about a week or two of that, then start asking for suggestions (give them time to get used to you) if they make one, think about it, and respond back after an apropriate amount of time.
Hope it helps