All the aerials we work with are dimensionally critical, and I don't think it's a good idea to play with any of them unless you have kit like SWR meters available.
The kit sounds like it's giving rather old fashioned instructions which were commoner in the 1960s-80s, when people were still used to making their own radio sets. In those days you might measure the length of the wire aerial you had on your 27mHz receiver, and if you had an 18" whip aerial, you might cut 18" off that wire and solder the remaining stub to your whip - thus giving you the same length as you originally had.
I never did this as I didn't want to run the risk of altering things, and it's pointless to do with 2.4ghz (as well as being much more dangerous, as HS93 points out). Good point by Netleyned, too. Incidentally, the actual aerial on your 2.4ghz is the little 30mm unsleeved bit at the end...