The resulting grid will have 22 ways to sum the called out number. I don't think there is an easier method using a geniunely called out number with fewer or simpler calculations.
The method described by Allerton (which is "rediscovered" in quite a few other books) is indeed very simple, but it can be improved upon. The method described in Mindsights
uses slightly simpler math and creates a square with 24 strong ways to add up to the (freely) chosen number (there are actually 26 ways, but two of them are somewhat less interesting, so I rarely point them out, in the interests of time).
In addition, the version in Mindsights
can be used with numbers of any size, which although possible with Allerton-type squares, tips off the method (try it with a larger number, such as 99, and see how "unusual" the square becomes).
But most important of all (especially to walk-around and trade-show performers) is the fact that the technique in Mindsights
can be repeated
. You can create squares for multiple people (even with the same target number), and they don't all end up looking alike.