The suggested ambient level produced by gensky equals the sky (indirect) horizontal irradiance times pi, and this is a suitable setting for -av (you just enter it three times for a gray RGB value). Do not use this -av setting for interior positions, only if you are rendering an outside view. Otherwise, you are liable to greatly over-estimate indirect contributions.
Also note that the importance of the -av setting decreases with increasing -ab bounces, but it never disappears from the calculation. Some would say that the safest setting for -av is 0 0 0, which biases the calculation on the low side, but will always converge to the correct value. If your -av setting is too high, then you will converge from a high value, but this can take longer if your setting is much too high, which is why you should not use gensky’s recommendation for indoor views.