Can I say my 2 cents: atmospheric absorption at 40kHz is a whopping 150dB/100m (20C 70%H) at nasty
10kHz it would be only 18dB. So for larger distances go down with your frequency. Also your calculation seems off. 100m means -80dB (inverse square law). Then there is absorption/diffusion at the reflector, let that be another 20dB. This makes it -100dB. Your receiver needs 94-75= +19dB SPL, you have only 108 - 250 = -142dB available. Forget it. You can concentrate your beam with an old satelite dish (not the offset ones) by mounting the transceiver in there. This would maybe improve the ratio by 30dB. But then any temperature gradients or air movement would carry your beam away. Figures are from "Sound System Engineering" by Davis&Davis, ISBN 0-240-80305-1