How to calculate the Sentinel-1 range resolution?

Hi all,
I am trying to calculate the Sentinel-1 range resolution for the IW High Resolution GRD product. According to the User Guide it should be 20 m, after 5 looks in range, i.e. after averaging 5 pixels in range so reducing the original resolution from something close to 4 m to 20 m. Now it is known that the slant range resolution r is given by the formula

r = c/2B

where B should be the chirp bandwidth (not the pulse length). Now, after a lot of search, I wasn’t able to find in the Technical Guide nor in the Sentinel-1 Product Definition the bandwidth value B to be used to get the 20 m value for the range resolution.

My question is: might it be the “Maximum range bandwith” ?

Its value in the SAR Instrument Technical Guide is set to 100 MHz that could result in a 3 m resolution that after the multilooking should give 15 m, close to the 20 m defined in the User Guide. Thanks in advance for any clarification.