Remember, pivitotal altitude is the altitude based on ground speed. If ground speed decreases, pivotal altitude will decrease and vice versa. You can use TAS in MPH or IAS. If you figure your Va to be 105 then take 105 (IAS) squared divided by 11.3. This will be the pivotal altitude, but if you have a tail wind of 20 knots, then your entry altitude, since you will be going downwind, will be higher. Figure out what the altitude will be for a speed of 125. THen the same thing for the lower altitude with a headwind, say 85 knots. THis will be the altitudes you will be hitting throughout the maneuver depending on wether you are in a headwind or a tailwind, or crosswind for that matter.
Just a side note...try not to go into your commercial single engine add-on checkride and have NO idea how to figure out pivotal altitude...He asked me what it was I said "800 feet". Needless to say he didn't like that very much. Good thing he was forgiving and took a bathroom break to allow me to figure it out.
You are right about the MPH using 15 as a division number. I was just refering to the IAS in Knots which I might have left out. Sorry if I confused anyone. I am preparing for my CFI checkride, so I need to keep things like that in mind.