Not only is it more complex than an exclusive tax but it is deceitful AND it calculates a tax on a tax. Plus it has to be back calculated unlike the exclusive tax.
If a $100 item includes a 20% tax of $20 that is saying that the $100 = item +tax = $80 + $20 but the 20% multiplier is multiplied times the TOTAL (item +tax). This is a ridiculous way of calculating a tax and no other tax is done this way. It in no way is more honest or up front unlike the exclusive tax calculation nor does it more clearly show the tax burden.
Either method of calculation can yield the correct calculation and neither is really more difficult than the other. Also (in some hands which shall remain nameless) either method of calculation can be used incorrectly to give incorrect results. Either way is honest and up-front but you are merely unused to thinking about it.
Income tax rates are actually stated on a tax-inclusive basis as one can see by doing some simple arithmetic. That is certainly a valid reason for using the t-i statement in the FairTax bill - to see how high your income tax rates are on a comparative basis. That helps put the FairTax rate into a comparative context.