If you want super precision, Step Sequencer modulation values can be edited in the Event List. As far as the Event List is concerned, the Step Sequencer is just another group of MIDI data.
Just don't be confused by the Event List "self editing," because it interprets MIDI continuous controller data properly. For example, if you enter several Controller 74 events in a row
with the same value, the Event List will show only the first one because a continuous controller represents only one value until changed.