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?
The fist business, the seed grower is taxed> the farmer that purchase the seed is paying tax on the tax the seed grower was taxed (also pays tax on his wheat harvest)> the flour mill that buys wheat from the farmer pays tax on the tax that the farmer paid on the seed growers tax. as well as paying tax an the farmers tax and also pays tax on his flour manufacturing, etc, through six levels. Basically what the spread sheet accounts for.