I'm not sure percentage of the tax break is quite the right measure. Someone with low income might have gotten a bigger break by percentage, but that would make little difference in their actual income. According to the data this author references, someone in the $40k-$50k bracket on the chart got an 18% tax break in 2018 as compared to 2017, a difference of about $500 over the course of a year. Hardly enough to make a difference in one's overall financial picture. Meanwhile, someone in the $5-10 million bracket got only a 3.48% break, paying, on average, almost $70k less after Trump.
Seems disingenuous to say that the working and middle classes got the most help when the upper brackets' tax break was worth more than the total income of someone in the middle when they didn't need it. It's not like you're rich if you get to keep $6.1 million, but no longer rich if you only get to keep $6m -- or as if that amount of money is the difference between whether you can invest in a company or not.
By another measure, the people making $10m or more got to split about $13 billion between 22,000 people while those making $40-50k had to split about $6 billion between 12 million people.
Just for comparison, here's another analysis:
https://www.policygenius.com/taxes/who-benefited-most-from-the-tax-cuts-and-jobs-act/