Here's the said chart :
https://www.reddit.com/r/ffxiv/comme...chart_instead/
Frankly speaking, I doubt it'll help more than trying to do the math.
I don't know how severe can dyscalculia be, but is it so bad that you'd get stuck with 1 digit numbers calculations ?
As for multitasking, boss isn't attacking, so you have like 10 second to place yourself. If they have trouble calculating, may be they should stop DPS and focus on the mech

?
Biggest problem with a chart, is that you need to get your eyes out of the game to check it.
Also, if you already have trouble doing 1 digit maths, I really doubt a chart is going to help.