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A three-dimensional calculation of amplified spontaneous emission (ASE) is presented that allows rapid determination (with suitable approximation techniques) of extraction efficiency and power gain for small aspect ratio (length/width) laser amplifiers and oscillators. These calculations include nonsaturable absorption, ASE line-narrowing effects, and end mirrors that reflect ASE flux back into the active gain medium. The applicability of this technique has been verified for laser aspect ratios below unity, and results are presented that show the optimum scaling of extraction efficiency and power gain for large fusion class lasers. The technique is particularly suited to first-order design efforts where fast turnaround results are required. Nevertheless, the approach shows an accuracy of at least 10 percent when the laser medium is efficiently loaded with coherent flux.