I think I have been misunderstanding the benefits, too, so I have taken brad813's and richmo's advice and done the math. I can't make it work for a solo traveler.
I called (waited for far too long...) and confirmed they take the 20% from the full price but not the tip - pretty standard.
A $60 meal - 20% = $48. The mandatory 18% tip is $10.80 so the total you pay for a $60 meal is $58.80. A saving of $1.20.
You pay $150 up front. $1.20 ÷ $150 is 125.
If you eat 125 meals at $60 you hit the pre-payment. So every meal after 125 you're saving about a dollar.
A $35 breakfast saves $0.70 - a $100 dinner saves $2. Can't see how one would break even on this.
Again taking Brad's advice I have decided it would not be worthwhile for me to have given my personal circumstances. I will not be eating 100 table service meals during the year. For me, I'll take the 10% AP discount on my $60 meal, add a 20% tip and pay $61.20 ($2.40 over the TIW). I'm not paying $150 for the discount AND giving a good server a good tip.
Now back to Disneylover1970's solo trip: Where are you staying?