From my experience, the leaving is a lot earlier than you first think! (not crazy amounts though, just slightly)
Going at it as close as the move allows DOES get you up there but, when your rear wheel hits the wall, you usually end up too leaned backwards which might be causing your inconsistency.
Leaving earlier should mean you gain a lot more forward momentum which, should help carry you on top of the wall once your rear wheel hits.
Hope this help! good luck