The amount in box 9a has no bearing on preparing your tax return. The amount in box 1 represents your share of the distribution that is income to you. However, the amount in box 9a should be the percentage that your box 1 amount represents relative to the total of the box 1 amounts of the Forms 1099-R issued to all of your mother's beneficiaries of this inherited IRA account. The percentage in box 9a should correspond the percentage listed for you on the IRA's beneficiary statement given to the IRA custodian by the deceased.
