From an ATC point of view, we don't like crossbleed starts on stand as there may be aircraft taxying behind which could possibly get blown over from the increased thrust of the engine used to provide the bleed air to the engine needing to be started
Not forgetting of course the increased danger to those PEOPLE on the ground!
the highest I've heard was 80% thrust needed to do the crossbleed (which I thought was rather a lot).
Although 80% sounds a lot you must bear in mind that something like the RB211 or CF6 on a 747 idles at around 62/64% N2 or N3. The figure is where you get max bleed pressure before the change over between IP and HP air.