I see you can't wait to start your next build! I am thinking the exact measurements you are looking for may be different, depending on the brand of aquarium you purchase. I have an 180g Oceanic aquarium but I also measured a 180g All-glass brand aquarium and noticed the trim on the Ocean aquarium was almost a half inch wider.
To be safe, 48.5 x 24.5 would do it, unless you want an exact measurement. If you go with All-glass, you could measure the thickness of the trim on comparable aquariums at your local fish shop for an exact measurement....I image they use the same trim thickness for the standard 125 and 150 g aquariums as they do on the 120g tank.