Recipe: Improvising cake flour

This is a quick follow up on the post regarding the calculations for substituting flours.  In my investigations, I came across a way to improvise cake flour using all-purpose and cornstarch.  Cornstarch has about a 13% protein level, which negates the whole protein math approach of mixing flours, but think back to the last time you made a slurry - adding water doesn't develop any gluten - it's not there, so it makes sense that it could be used in a pinch to lighten up a wheat flour.  Case in point:  my favorite shortbread recipe uses a lot of cornstarch mixed with the flour and the result is very light and crisp.  The usual method to improvise cake flour this way is for every cup of all-purpose used instead of cake flour, subtract 2 tablespoons of the flour and replace it with 2 tablespoons of cornstarch.  Of course, if I'm forced to make a substitution this way, I'm going to do it right and at least weigh it out (I would have bought the cake flour to start with, but say I ran short), so here's the conversion:

For each US cup of flour:

 - first off we will accept 125g as the metric weight of a cup of all-purpose flour

 - a tablespoon of all-purpose weighs 7.8g, so we'll take out 15.6g (just weigh out 109.4g flour to start with)

 - a tablespoon of cornstarch weighs 9.4g, so we'll add in 18.8g cornstarch

 - sift 2 or three times to ensure a good mix

 - use as the recipe dictates