ORIGINAL: Rodney
Xjet is right in that cutting the radiated power in half doesn't just cut the range in half, it cuts it by 4 (down to 1/4 normal range).
You've got that a bit backwards actually.
The inverse square law says that to double the range you must increase the power by a factor of four.
Hence, halving the power will reduce the range by a much smaller amount than you'd actually expect and certainly not the 75% reduction you suggest :-)
I'm too tired to do the math but a 25% reduction in range is probably closer to the truth.