you need to allow enough time for the transfer to Falmer so it reaches there early enough so that you don't miss kick off.
Please be aware there may be others there also wanting to use the same train.
So to be sure i would say 4pm.
Based on last time we placed Palace at home I would allow an extra hour if changing trains or getting on at Brighton. In fact I would walk to London Road station and pick up train from there as probably quicker than all the malarkey at Brighton station with police segregating Palace fans and the resulting queues to even get on the platform. Remember that not all trains stop at London Road so not such a choice of trains. I ended up missing kick off last time because I didn't allow enough time.
As quick as you can. Get there in loads of time, down a pint of courage, get involved with a pie and watch some TV coverage of us SMASHING them on ST Patricks Day.
Less stress - more relaxing - more £££ for the club! Everyone is a winner.