is the range measured from the unit leader with standby to a target or from any figure in the unit with the stand by?

The range to trigger standby is measured from any figure to any figure in both units (the one triggering standby and the one doing the standby action).

Pretty much the only time you measure range from the unit leader is for attacking. Measuring from any model in the unit is the "default" way to do it, and attacking is the exception (even though you probably do it that way more often given how common attacking is.