No one can predict exactly how large the crowds will be or how early you should go to find a seat for Fantasmic! on any given day.
It has totally to do with the whim of the crowd at DHS on that day, and the general crowd LEVELS.
How much time you wish to "invest" in waiting is a personal decision. We never do the formal Dinner Package, and have always seen the show (dozens of times).
(Take all of these as "average" numbers, sometimes the crowd will be MUCH greater and the theater can fill up much sooner.)
VERY Busy days:
If you go to the first show of the evening (or "only" show that evening), if you arrive about 75-120 minutes before show-time, you can sit just about anywhere.
45-60 minutes before: Good seats
20-40 Minutes: will vary between seats at edges and rear of theater to SRO or even "closed" to further guests.
Busy days:
If you go to the first show of the evening (or "only" show that evening), if you arrive about 50-60 minutes before show-time, you can sit just about anywhere.
40 minutes before: Good seats
25-35 Minutes: will vary between seats at edges and rear of theater to SRO or even "closed" to further guests. (The crowd of "last-minute" arrivals can be slow to get seated before show-time.)
Off-Season (not busy days):
You should be OK to enter by 10-20 minutes before, but you'll likely be seated to the outer edges, and/or way at the back.
If you do the Fantasmic! Dinner Pkg. you still SHOULD enter by AT LEAST 35 minutes (much "safer" at 45 minutes) before show-time and still be guaranteed "pretty good" seats (not my "first choice" area, but fine), regardless of crowd-levels. Remember, the DP is only valid for the first show of the evening, if there are 2 or more. If you do have DP and you spy some seats outside the DP "reserved" area, you are free to choose to sit anywhere outside that area you like, no problem.
If there are TWO Fantasmic! shows in one evening, the second one will have lower crowd level than the first one. You can arrive a bit later and still find a seat.
Be aware that the second show waiting area will LOOK like the show is "too crowded".
That's because they HOLD the 2nd crowd outside the theater while the CMs CLEAN it after the first crowd has exited.
Then, they will let those waiting together in that big group IN... all at once, and there will be PLENTY of room.
(The theater seats over 7,000 guests.)
When you enter the theater, there will be CMs along the way directing the flow of traffic, but you may (if you are early enough to have a choice) simply choose any open seats in the theater to be your own.
If you want to exit more quickly, choose a seat farther back and to towards audience-right.
If you want to be immersed in the show (and sometimes, immersed in water-mist, if the wind is blowing "in") choose a seat towards the front.
We like to eat our evening meal while waiting for Fantasmic! to start.
You can bring in items from the counter-service places in MGM, or they DO have concession stands at the rear of the theater serving "baseball stadium-type" food and drinks. Yes, even beer, sold strictly ONE-at-a-time, one-only-to-a-person, so a single person can't go and buy beer for other members of a group... all guests wanting to buy a beer must be present in-person at the counter.
We have a hot dog, maybe a pretzel, later some popcorn or cotton candy (Disney Health Food).
http://www.allearsnet.com/menu/menu_fan.htm
Click above for the "menu."
As an added bonus, of course, you're that much closer to the exit afterward. 
. Actually, I wouldn't mind not sitting up close at all, as long as I can see the stage area, I wouldn't care.
