So I'm really looking forward to getting the intermediate course up online soon which will be better examples and demonstrations on video for you. Basically, when you're doing sirens you really don't need a piano exercise, just make a sound and siren up and down through your range the "Ooo" sound is also a good one to use in head voice. The cool thing about the sirens is that you don't really need a piano exercise, meaning you can do these at any time any place. As far as what pitches to siren to, a half scale like the 5 note pattern or a full octave like a major scale from C to C. But again there is no one pitch that's really necessary. Just getting your vocal cords to stay connected as you vocalize up and down through your range. I hope that helps?
Keep singing,
TV Jim