<PAGE>   1

                                                                   EXHIBIT 10.15
INTER
OFFICE
MEMO


                    DATE:  January 15, 1999




TO:             Mr. Keith Hull

FROM:           Sharon L. Rodgers


         I am pleased to inform you that you have been approved to participate
in the management incentive plan for fiscal 1999. You will participate in the
plan based on an individual percentage award of 50%.

         For fiscal 1999, 75% of your actual incentive award will be based on
achievement of Financial Goals, as set forth below, and 50% of your actual
incentive award will be discretionary, as outlined below.

                                 Financial Goals

         The Financial Goals for your fiscal 1999 award are as follows:

<TABLE>
<CAPTION>
                                          Corporate
                                          Operating Earnings
                  Performance             (Thousands of Dollars)
                     Level                       (EBITDAL)
                  -----------             ----------------------
                  <S>                     <C>
                     30%                         101,000
                     45%                         109,000
                     60%                         117,000
                     75%                         125,000
</TABLE>

         75% of your actual incentive award will be based on achievement of
Financial Goals. Your actual incentive award will be determined by multiplying
your total base salary paid in fiscal 1999 times your individual percentage
award times the Performance Level achieved as set forth above and will be
calculated by the Company. Any such award will be prorated when the Performance
Level achieved falls between the 30% and 75%

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levels. No incentive award will be paid if the Performance Level is below 30%,
and no additional incentive award will be paid for any financial results which
exceed the 75% Performance Level.

                                  Discretionary

         50% of your actual incentive award will be based upon successful
progress, as determined by the Chief Executive Officer, towards objectives
established between you and the Chief Executive Officer. The award will be up to
50% and may be prorated at the discretion of the Chief Executive Officer.

         The general guidelines of the plan for fiscal 1999 are as follows:

         1.       Corporate Operating Earnings (EBITDAL) will be defined as
                  reported in the Company's financial statements.

         2.       Your actual incentive award will be paid on an annual basis,
                  with payment to be made as soon as practical after the fiscal
                  year audit is completed.

         3.       No incentive award will be paid if you are not in the employ
                  of the Company on the last day of the fiscal year (effective
                  date of severance must be after fiscal year end).

         4.       In the event you are reassigned or given a new position and
                  responsibilities during the fiscal year (promotion or
                  demotion), payment of the incentive award will be determined
                  on a case-by-case basis. The Company reserves the right to
                  deny payment or to reduce the incentive award in the case of
                  demotions.

         Please indicate your understanding of the plan, the performance goals
and your potential incentive award by signing and returning this letter to me. A
copy is provided for your personal files.



                                                   \s\ Sharon L. Rodgers
                                                   ---------------------
SLR/kjw



Acknowledged: \s\ Keith M. Hull
              -----------------

Date:           January 22, 1999
                ----------------




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