The gausian definition is used for the sampling scope, so the formula is: T_rise (10-90%) = 0.339 / BW. In other words, a sampling head with a 3dB bandwidth of 30 GHz would have a rise time of 11.3 ps (for 10 to 90%). For the 20 to 80% risetime the formula would be: T_rise (20-80%) = 0.223 / BW, which is 7.43 ps rise time.