RendoasI can add 20s to 30s because I know that 5 + 3 = 8, but if I have to add 20s to 55s and have to think about the sum rolling over into the next minute--but wait I'm supposed to subtract a minute--but I'm in the next minute now--I can't get my brain to work fast enough. I know this is objectively simple, I just cannot do it quickly. I probably could if I had a pencil and paper to write it down and look at it for a bit.
I'm not 100% sure about how dyscalculia works, but what if you think of it not as addition/subtraction, but rather memorization? Using hooli's method, you would be adding the same 20 to every situation. So instead of thinking on the fly what is 30 + 20, you could make flashcards to memorize 00 + 20, 10 + 20, etc. That way, when you see 25 seconds on the clock, you have the 2 already associated -> 4, and then the ones place stays the same, so 45.
Beyond that, a lot of tracking ubers is a vibes thing. You know at the beginning of a push that you have a certain amount of ad (because you were at 70% when their medic spawned, for example). Then, since it's big ad, you decide to milk the uber. Then after you get that point, you ask yourself "was that a slow push, or did we go fast?", and from there, you can make an educated guess on if it's still ad or not. As you get better, your mental clock on knowing how fast you're going will get better and more specific. Then you can incorporate other factors, such as where the enemy team is positioned, if you've been building well, spotting the enemy medic's uber with the solemn vow, etc). Make it a habit to ask yourself the question "What are ubers?" and then you don't miss any details. And at the end of the day, everyone on your team should be able to track uber at least to some extent, so talk with your teammates (i.e. "Do we still think it's ad?") and decide from there.