Its covered on the IRS website - the old system is the capital gains tax
it gives a list of the allowances which I've cut and pasted below - you can see why the IRS couldn't give a definitive answer to the tax payable - there is an example on the website as well
"What deductions can I claim to calculate the capital gain under the old system?
To calculate the capital gain under the old system you can claim the following deductions:
the price at which the property was acquired;
the inflation element;
any ground-rent paid on the property and for which a deduction has not been already claimed in any other way;
a maintenance allowance at the rate of 0.4% for every year that you owned the property;
improvements carried out;
any duty paid on acquisition;
other expenses directly related to the transfer but not exceeding 5% of the selling price."