From ZDoom Wiki
40:Ceiling_LowerByValue (tag, speed, height)
- tag: Tag of affected sector
- speed: How quickly the ceiling moves
- height: Amount to lower ceiling by
Lowers a tagged sector's ceiling by height units. If tag is 0, then the sector on the line's back side is used.
ceiling_lowerbyvalue(30, 8, 96); //30 is the tag of the sector whose ceiling will be lowered