Those two point are your bounds, anything within them will do the job. If the ceil of the result or the floor of the result is within the bounds use that. Otherwise you will have to go with some sort of a decimal.
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if ( ($x / ($aa - 1)) > $z )
echo "<br><b>Rise Height greater than max allowed !!! </b><i>(". $z . "mm)</i>";
echo "<br> Using one extra step (". $aa .") would give a Rise height of : ";
echo round($x / ($aa),2) . "mm";
echo "<br> But that is below the Minimum Rise height of " . $y . "mm";