Adjusting Minn Kota Endura 30 to 15" spec

Hi,

I have a Sevylor HF280 inflatable. The directions suggest
only using a motor with a 15" shaft. The MK is 30". Where
do I measure to adjust the MK shaft down so that it would sit
in the water at the same depth as a typical 15" length motor,
or in other words the Sevylor motor that they sell.

The obvious answer might be at 15" I assume but I'm not
sure if it might actually be a bit lower since I would think
even with a 15" motor the attachment to the transom might be
lower than the very top of the whole unit.

Thanks,

Jesse