Hi Sail Quick,
OK, you can extend your boom to 240 cm and use your sail with a 238 cm recommended boom length.
What do you do if the wind comes up....... is 2cm of additional outhaul going to "depower" your rig enough?
What if you want to downhaul your sail alot more..... you have only 2 cm more boom length? You may not be able to get any outhaul (to stabilize your sail) at all as the clew length will have "grown" to well past 240 cm.
As Geo suggests, it depends on the boom.
I have some booms I run nearly fully extended, but when I'm out on them I don't feel real secure as they bend, flex, creak, etc.
I have other booms that just feel soft and flexy once I extend them beyond about 75%.
So, if I'm sailing alone, or far offshore, or in a race, I'll use a boom that I know I can trust and I won't extend it to the max.
I've broken booms and had to sail the unbroken side back to the beach, or had to swim back in 36 deg. water and 40 deg. air temp.
Don't really want to do either of those "self rescues" again, so I'll "err" on the side of caution when it comes to boom extension.
Hope this helps,