HP1 is derived from the "% Engine load" gauge. Put "% Engine load" along side HP1.
Also, there was a bug in UltraGauge EM's, prior to version 5/8/12, that affected HP1. Check your version date:
Menu --> UltraGauge Setup... --> version
FIX: In order for the KW1/HP1 or TRQ1 gauge to function, the “%Engine Load” gauge must be placed in the next display position. For example, If HP1 is at position 2, and TRQ1 is at position 3, then % Engine Load must be placed at position 4. Also, TRQ1 and HP1 should be in adjacent positions, i.e., 1 & 2, or 2 & 3, or 3 & 4, etc. (This only applies to versions prior to version 1.2. This requirement was removed on version 1.2)