The smartest way to calculate a tip

I once had a girlfriend in Honolulu, who was smart about math, especially calculation. According to her, she used to learn Japanese Abacus from her father. So she was good at sums in her head.

One time, together she and I went to the restaurant and I learned something unforgettable.  It was the easiest way to calculate a tip. It was more than shocking because at that time, I was using a small calculating-table or calculator with me.

First, she taught me to calculate 10% of total cost.   And she explained that all you need to get number of 10% was just move a decimal point.

For example,

10 % of $23.6 is calculated by just moving [.dot] to the left side.  So it will be $2.36.

In like manner,

10% of $32.5 is $3.25.

10% of $46. 67 is $4.667

10 % of $78.29 is $7.829

10 % of $136.8 is $13.68

10 % of $284.2 is $28.42...

After getting number of 10%, she said, "Do double that number" which is 20%.

20% of $32.5 is........(10% $3.25) x 2 = $6.50

20% of $46. 6 is.....  (10% $4.66) x 2 = $9.32

20 % of $78.29 is......(10% $7.829) x 2 =$15.658

10 % of $136.8 is .....(10% $13.68) x 2 =$27.46

10 % of $284.2 is......(10% $28.42) x 2 = $56.84

I told her, "To me, it was a little difficult to calculate double."  And she replied, "It was not necessary to calculate a tip exactly.  "About" was always good enough as for calculating a tip.  For example,

10% $3.25 About 3 x 2 = $6   (About 20%)

10% $4.66 About 4 x 2 =  $8   (About 20%)

10% $7.829 About 7 x 2 = $14   (About 20%)

10% $13.68 About 13 x 2 =$26 (About 20%)

10% $28.42 About 28 x 2 = $56 (About 20%)

Since then, my calculation of tip became so easy and I learned something important from her. I also learned tipping was an expression of gratitude.  The more generous you are, the more people could be generous to you too.  Therefore tip is better to be "about" $2020%....if possible. 

Honestly speaking, I do miss the dating with her, but I don't miss her any more....because she is now my wife.