Ilanfairpg, Granted, but the important point is never to cross an active circuit crosswind at a point that traffic taking off will have arrived at.
If you look out the window and have a flexible plan you never will. Using the method desrcibed by some on here you have to cross at the numbers so if the aircraft that is taking off reaches your height and position over the numbers you collide, using my method you are not constrained to a particular crossing point only by the traffic you can see and hear. Besides how do you know what height an aircraft will reach by the end of the runway, too many variable factors here to make a sensible early judgement.
Upwind runway joins are quite permissable in some situations, your join is constrained by the ATZ not by folklore
Its bad judgement to cross a runway centreline at any point when an aircraft is below you and climbing on that centreline. Its completely avoidable by early planning and not sticking to a infelexible rigid course