I figure the longer my jump, the higher the arc must have been.
Does blade legnth matter? Is it a good indicator of the jump's power? Is it an indicator of being able to move onto doubles? What is the "blade length" of doubles. We have a strong junior skater who is my height, and her loops (even singles) are well over 4. But, I haven't measured her.
Neverdull.
You have to be very careful with this thinking!!!
A longer jump is not necessarily a technically clean jump. In doubles yes you need the height (to make it clean and pretty... I've seen girls land clean double by hardly jumping), but the height doesn't come from jumping out. It comes from jumping up, and the double rotation comes from a quicker snap not necessarily raw power or speed. In fact, a lot of coaches teach doubles from a near standstill to make sure that skater can generate enough rotational velocity to get the jump around.
I often get the comment from my coach, " You are jumping to far out" specially on my 2Sal and most of the when this occurs it ends up with me on the ice. Actually, most of my doubles are "smaller" than my singles. My Single loops and Sals are around 4 blades long, while my double loops/Sals are around 2 to 3. Anything longer and I know I jumped to far out to snap in quick enough to rotate them or pre-rotated my shoulders. My Double Toe attempts are problematic because I don't pass at all, but the jump length is huge because I take often too much speed into them, but I have yet to land a clean one.
Personally, I think once you are more comfortable with the technique of the jump, the jump becomes large/height. So, I would never suggest waiting for a particular jump length before starting a double. I think the technique is more important.