sky9
I think you mean VA not JA since that is on 24R!
Full length of 24L is from T1 but use of this distance requires ATC to operate in dependent mode ie inbounds become a factor.
When there are inbounds to affect use of T1 for departures, then you will either be kept on the ground until the arrival has landed or be requested to taxy forward to abeam VA (where the white runway edge painted lines start is far enough), not
VB where the colour change takes place and the piano keys are.
Departures from abeam VA or further west allow the runways to be used without reference to each other. This is in order to comply with ICAO criteria for operations on closely spaced parallel runways.
If you are strapped for performance and need the T1 length, then just ask and you will receive no questions asked.