the train departs at 23:25, so you should attempt to reach by 22:30, which means arriving at dadar at 22:00
However, knowing how the traffic in Mumbai is, you must try to reach dadar by 20:30 or before, which means taking a bus that departs atleast 4 hours before. Deduct one more hour to account for delay faced by bus on its round trip.
16:00 hrs, in my opinion should be a good time to depart, and at any cost you must be on board at 17:00. for first few trips, since...
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you are new try to depart on or before 16:00. Once you know the way well you can try optimizing the schedule.
and in monsoon always keep high buffer, as the roads tend to get horrible. Even now, if you travel, you'll find potholes on Sion Panvel highway itself, with traffic jams being common at Belapur flyover and Vashi gaon area simply due to a really bad pothole ridden stretch which is barely 100 metres long!! the roads are probably problematic even beyond Vashi too but i don't travel on that side (My route involves leaving sion panvel stretch right at Mankhurd station)