I assume the line 18 is required because you must guess at the breakdown of SE activity $ for the COVID period (after March 27) in order for Turbo Tax to figure the max deferral. The max deferral must be figured anyway, and then it is YOUR choice to enter (some, all, or $0) amount you wish to defer. Enter it on Line 11 schedule 3 Part II, and Turbo Tax will take it from there, when you close that form, as the changes will cascade into appropriate related areas. For more info, I defer to user adamclerman.
