You don't have to change your Display Units, it's just that the ISF shader expects its parameters in inches.

If you want to enter these four parameters in feet instead of inches, you need to change the three values in the P node from 0.0969 to 1.1628. According to bagginsbill, the P node returns its position values in one-tenths of a Poser native unit, equal to 10.32 inches. This needs to be converted to whatever units you want to use in the parameter nodes.

Note to bagginsbill: while checking and double-checking the corrections to my version of the ISF shader, I have come across various versions (all posted by you) with different values in the distance Pow node and in the final Divide node. Can you confirm which values are correct, please? I don't want to post a thread with a corrected version if it's still wrong. Thanks.

(edit) the version I used for the above-posted correction is shown in the image above. The only difference is I did not round up the P-node conversion factors to 0.1

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)