I tried that calculation for a married couple last year, each earning over $100k. I even watched the darn video (I HATE VIDEOS!), or part of it anyways. After 1 hour I still couldn't get to anything that made sense.
I also spent 2 hours spreadsheeting it (I do love spreadsheets), and still couldn't get it right.
Like you, I knew how much they needed to pay for the year. I'm not sure how anyone can make sense of the simplified(?) method, I felt like an idiot.
So I went back to the old tried and trued Irish method taught to me by my first mentor. It takes 30 seconds or less. EVERYONE, regardless of age, income or marital status is Single, zero dependents, plus $50 (or $100 or whatever) extra withheld. I tell my clients that they MUST have me figure their W-4 if they change jobs, and I then send them a preprinted one to give to their HR department.
The result? Next year they will get refunds (If not, add more to the plus $50), they will be very happy, they will not take up endless hours, emails and phone calls asking "Why didn't I get a refund?". They tell their friends you are a genius, and you can now raise their fee.
If they complain about the extra withholding, and that has only happened twice in 30 years, I tell them it is just temporary to catch up on their underwithholding year to date and we will adjust it next year, by which time they have forgotten all about it.
It is always easier to get someone to "Try" something than to "Do" something, so my conversation always starts with "Let's try this...."