The user first launches a kunai with an attached PMYF seal loaded with its maximum carrying capacity (currently 100kg) into any direction. Then, the user exchanges position and momentum with the seal's load as soon as it's released.
Because the seal's load needs to be in substitution range (meaning it could be reached by "normal" means; see the Substituion page for details), it is ideal for the load to be unstored as soon as possible.
Because the seal will be very close to the ninja, the position-exchange component of substitution can be largely ignored.
The unstored object, of mass M, will have the same velocity v as the seal attached to the thrown kunai.
If the ninja's mass is m and the ninja's post-substitution velocity is v', then Mv = mv'. This means that v' = v * M/m.
In typical use by the players, the unstored object will have a mass of 100kg (the heaviest that can currently fit into one of Hazō's seals), and the user will weigh less than 100kg. It's reasonable to assume that ninja can throw a kunai at ~60 m/s; this is less than the Earth-human record for baseball pitches, and ninja are stronger than Earth-humans. Assuming a body mass of 66kg (because it makes math easy!), they can launch themselves at ~90m/s. Launched straight up, this allows them to reach a height of ~400m... in a vacuum. Air resistance becomes a significant factor at those speeds, and I can't estimate it well enough to say how high they'd actually fly.