STEEP DESCENT and UNABLE DESCENT PATH

Hello all,Just wondering what parameters the 737NG uses when deciding whether to display STEEP DESCENT AFTER ABC or UNABLE DESCENT PATH. Is it a set angle of feet/nautical mile? Just curious, because I am creating an approach for Penticton, BC which calls for a descent of 4,371' in just 7.6 miles (beacon altitude to touchdown elevation). That is 575'/NM. It did it, but warned me of the STEEP descent in advance.One thing I am doing to "help" the aircraft achieve these descents is to slow right down to 160kts with a programmed speed constraint prior to the steep descent. I'm not sure if this is a realistic thing to do or not. I would like to make my procedures as realistic as possible. Perhaps in the real world, VNAV would not be used for such a steep descent angle?Roger Precious