35 ILCS 405/11
If the person who pays the Illinois transfer tax arising from a taxable transfer is entitled under the Internal Revenue Code or any other state or federal statute or rule of law to reimbursement of a portion of the federal transfer tax from any other person who has received transferred property, then, unless the governing document directs otherwise, the person who paid the Illinois transfer tax shall also be entitled to reimbursement from that other person of a portion of the Illinois transfer tax. The amount of reimbursement shall be determined by multiplying the total Illinois transfer tax by a fraction, the numerator of which shall be the gross value of the transferred property received by that other person and having a tax situs in this State which gives rise to a right of reimbursement of the federal transfer tax, and the denominator of which shall be the gross value of all transferred property having a tax situs in this State.
35 ILCS 405/11