Just averaging the shallow and deep end depths is usually close enough. In your case, (3 +8)/2= 11/2= 5.5 or (3+8.5)/2=11.5/2=5.75.
So if the deep end is 8 feet you have, 18*42*5.5*7.48=31,101 gallons.
While if it's 8.5 feet you have, 18*42*5.75*7.48=32,515 gallons.
Honestly, you can probably just use 30,000 for simplicity's sake and be just fine.
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