Centering framset

I'm having trouble centering a frameset (3 vertical frames with
content in center frame) under a seperate top horizontal frame (for
navigation). This top frame is Aligned center under properties. In the
bottom frameset I cannot use the allign center feature because the
content is created & modified using Microsoft Excel and saved as a web
page (.html) often, the allignment is never saved under Excel, which
is why I've had to sandwich the content between frames. You can view
the page at:

http://www.the-scenic-route.com/i-rentals.html
(Navigate to Rentals->Scenery)

On the bottom frameset the left and right frame columns are set to 10
percent, while the center is set to 768 pixels. I feel as if I'm
close, but it doesn't quite line up yet. Any suggestions? Thanks.