Exactly. I assume that the pool bottom has the same depth across the width of the pool at a particular point along the pools length. My pool has a 3 foot shallow and 6 foot deep end. Average depth (deep+shallow/2) = 4.5 ft. When I measure every foot , sum and divide by the number of measurements I got 4.1 feet. That's a 1200 gallon difference - about 10%. I also have curved corners where the bottom meets the sides. Its about a 2 foot radius, so I figured the surface area of the curved triangular area ((4x4)-(2x2x3.414))/4). Think of a circle inside a square of the same size. Multiply this area by the circumference of the pool to get cubic feet and multiply by 7.48 to get gallons. For my pool this adjustment was 51.5 cu. Ft. And 385 gallons (a subtraction). You can adjust for steps, swim out and other areas like yours in the same way. For my 13,000 free form pool it took 277 gallons to raise 1 inch - about 30 minutes with a garden hose. Being off ( of raising the water 1 inch) by 1/16th equals a 6.25% error - about 800 gallons. You can increase your accuracy by adding 2 inches, or 6 and adjusting the calculation. Don't know if its possible or desirable to lower the pool below the wading area then add 1 inch.