I've woken up annoyed and stupid but need to not be today

[WokeUpAnnoyed]

We have a visit from a higher up today. It's quite an important visit and determines the future of the site. We were sent a list of questions and i went through them with my boss last night. There was one question I admittedly should have known the answer to and didnt.
Previously always been ~told~ this. I have asked former managers how this is calculated/known but 1st manager never discussed numbers. 2nd manager used to crow about knowing everything and gate kept information to make themselves look better. Current manager has told me to go find it.
Not knowing is going to make me look stupid today.
I cant find it. I have looked on all reports for hours last night at home. Not knowing the exact way it is calculated means I don't know if I am looking in the right place, what numbers I need.
I now feel stupid.
I have worked over and above for this company and I feel stupid and incompetent. It isnt like I havent asked, repeatedly, how to get this information.
I have even asked the higher ups how to do something and was told "its just a feeling" wtf?
I then get told I am not ready for the next step. But if people gatekeep then I will never be ready. I am willing to learn and once told how to do something, I can.
I now have to make myself feel stupid again by asking how to do something or for the information directly because I couldn't find it, minutes before the meeting. Which will piss off my boss as they have better things to do with their time than 1)my feelings and 2)me failing to find it. So they will tell me the info, but I wont have time to understand how we got it.
So I'm gonna feel stupid and boss is gonna be annoyed.
I need to get out of this funk.

Based on the information given I would say the number is the same as last year.

Then I would stay silent until asked and at that point I would say-

As you know the process of getting to this figure has not, despite my asking, been forthcoming. I am ready to understand this process and look forward to spending some time with X to gain this valuable knowledge. When can we schedule
this?

To me, all their replies meant that they also didn't know, and didn't want to admit it, which means it was probably supplied by someone else, or some formula in a spreadsheet or database that works it out, that possibly no one even understands any more. Never underestimate peoples need to not look stupid, and lie to cover it up. If asked I would give the general meaning or terms used, and if asked for more I would advise the current calculation method is under review due to a lack of documentation on the method.

Higher up wouldnt know if have asked and I wont throw boss under the bus as it isnt just them, it's also been previous managers, so when the stakes are so high, it wouldn't be the right thing to do.
I will ask boss and take the feeling of foolishness. Then deal with the situation afterwards. I would love to be as upfront as some of you but it would come out wrong. I would rather feel stupid I think than have my boss think I am being a dick.

No, they all know. They can just spout it at will. In the meeting I got another number 0.05 out and was corrected. Why I couldn't be told this other one I dont know! Well actually, my boss is great and uses it as a learning method, which is great, but I feel so stupid when I cant get the answer.

They can also do the guesstimates for the other number too. I've seen them do it and be so close the the final figure it's scary.

I want the calculation as then i would understand the number, rather than it just be a number that inevitably falls out of my head as we refer to it so little. It doesn't impact our day to day operations, but it is a metric we should know.

It's my own stupidity making me annoyed, rather than at my bosses.

Thanks for the advice.
I actually spent hours on it last night. I am also in the carpark at work an hour early so I can go in and prep. I am just finishing my cigarette.