What you're thinking is correct, you're just getting confused.
If the stat stages of any of the holder's stats are negative, it consumes the White Herb and all negative stat stages are reset to be neutral.
Source
The above applies to any situation when your stats are below 1x. I tested to confirm this through a battle also. Sceptile's stats returned to 1x after the SpA drop from Leaf Storm, even when its SpA was earlier 1.5x.
White Herb (and any clone item) would restore all negative stats to 1x.
Edit: okay
When used from the Bag during battle, it raises the Special Attack of a Pokémon currently in battle by 2 levels. It also increases the Pokémon's friendship a little.
Source
As it raises the SpA stat by two levels, it will negate Leaf Storm's SpA drop. The SpA will restore to 1x. To answer your question, yes you can use this to counter the moves' SpA drop.