$top    = $page * 20 - 20; 
$bottom = $top + 19; 

##</code><code>##

$top    = $page * 20; 
$bottom = $top + 19;

##</code><code>##

$top    = $page * 20 - 19; 
$bottom = $top + 19; 

# A clearer Alternative: 
$bottom = $page * 20; 
$top    = $bottom - 19;

##</code><code>##

$top    = $pages * 20 + 1; 
$bottom = $top + 19;