L6 sale to consumer = sextuple taxation.
Not sure what you mean. If you studied the example you'd see that the l6 figure for "tax cost as % of sell price" was the was the amount of accumulated tax that had cascaded and embedded itself into prices at L6. It is not, however, a tax of that level (or any other) six times.
L6 is intended to be the level at which the thing is sold at retail. It is the (hidden) tax paid at that particular level expressed as a percent of the selling price and represents the potential for lowering prices by that amount. Keep in mind these numbers are merely examples and I'm not trying to say that these are any sort of definition of how much prices will be affected. My interest is in showing the mechanism.
Anyone wishing to can make his own copy of the spreadsheet and by selecting "tax rate" and "net profit %" can derive a L6 number (or even use more or fewer levels) that he believes is representative of the actual situation.
Does that clarify anything for you?