Your setting of ONDEMANDGPHDEV is ok. However, if your PCU utilization is more than 75%, then your setting of 25% will not work as desired, so check your levels of PCU utilization.
Also, in your earlier post, you said you’ve activated 2 fixed PDCHs and 1 on-demand. How did you define the number of on-demand timeslots? I thought their no. was not fixed, but varies depending on the data usage level in the cell?