another flexwatt ?

so i have a 20l tank for my ball, im having to move my tank to a slightly cooler location which means i have to add another uth. im wondering if i need an 11" x11" piece of flexwatt or would the 4"x11" be ok?

I have a 20L tank with a 12"x11" section and it does the job just fine with t-stat. Can't comment on the 4" though, in a cold room it "may" not be enough, especially with a glass tank. Never seem to hold heat very well.