Value checking
First CFO versions were using an extension called CHK to perform value checking in client-side (in client’s browser, using javascript)
That technique is obsolete and insecure due the simplicity of override it in bad hands.
So now we are reccomending to use server-side checkings, and those are some samples on how to perform it.

Simple conditional with a message to the customer

//
// Some dummy preparations
$meWidth = {12};
$meHeight = {14};
$SquareMeters = $meWidth * $meHeight;

//
// Perform server side checking
if($SquareMeters<1) { 
    CFO_AbortFormula("Minimum square meters is 1"); 
} else if($SquareMeters>100) {
    CFO_AbortFormula("Maximum square meters is 100");
} else {
    //
    // All ok
    $FinalPrice = $SquareMeters * $Price;
}