Do I have to take ro-accutane exactly same time everyday to increase the effect?

If the dosage of Accutane is twice a day, if you take it
between 6-7:30 pm each night that is completely fine. Precise timing to the minute
should have no significant impact on your treatment.

You can take it with the window you are. It's more important to make sure you are taking it with food, and with the fattiest and most caloric meal of your day. For you, this is clearly dinner, and it's fine if that's within a 90 minute window timeframe.