-40 is a great signal. Most radios will self adjust to signals -40 to -50 (these are the optimum signal levels). Signal levels in the -20 range or stronger could actually damage the radios. The limits (threshold) are normally about -65 to -70 before the radios will downshift modulation schemes. Worst signal is normally around -80 to -90 (and will have major problems if noise is present).
RSL = Antenna1 + Antenna2 + Power output - FSL (free space loss) - Other (connector, line loss, environment)
FSL = 20logd (KM) + 20logf (MHz) + 32.44
So for example, lets say you have a radio, 5.8 GHz, 16 dbi antenna, 10 dbm output, 0.5 km link.
FSL = 20log(0.5) + 20log (5800) + 32.44 = 101
RSL = 16 + 16 + 10 - 101 = -59 (if the threshold is -70, you have 11 db of "fade margin")