this is a fun mental exercise, it will be a disappointment when you finally fill the pond and measure the true volume

.
I don't know about the calculator you used on that web site; it doesn't work in Firefox, and my aversion to running Explorer is almost religious. But I digress.
Re-estimating given your measurements:
- if the pond were a simple rectangle, it would have an area of 8' x 15' = 120 sg.ft
(a 4' deep, square-sided swimming pool this size would hold 4" x 120 sq.ft x 7.5 gallons/cu.ft = 3,600 gallons. this is a large upper limit on the actual pond volume.)
- if the ends were perfect 8' semi-circles instead of 8' squares, you would lose ~1/4 x 64 sq.ft = 16 sq.ft; however, your design is a little squarer than that, so suppose the curvature costs only ~10 sq.ft., leaving 110 sg.ft of surface
- from your measurements, the inward curve which makes the pond kidney-shaped is less than a 1' reduction in the width, so that also only costs at most say ~10 sq.ft or area, leaving us probably a little over 100 sq.ft
- throwing in ~5 sq.ft for the big first step of the stairs eliminates most of the shallows and leaves 95 sq.ft of "pond."
- 95 sq.ft x 7.5 gallons/cu.ft = 712 gallons/ft of average depth
- If the rest of the pond is 67% @ 4' and 33% @ 2.5', then the average depth is (.67 x 4') + (.33 x 2.5') = 3.5'
- 3.5' x 712 gal/ft = 2490 gallons, plus the (15 sq.ft + 5 sq.ft) x 1' depth of the first step and the plant ledge = 150 gallons gets us back to ~2,550 gallons
which is basically where you started long ago when you first conceived of your design.