.htaccess RewriteCond for REMOTE_ADDR while behind Load Balancer?

I have a web server behind a load-balancer.

I need to add a conditional redirect to my .htaccess in order to display a maintenance page whenever we take the site offline for maintenance. This part is straightforward:

RewriteEngine on
RewriteCond %{REQUEST_URI} !/maintenance.php$ [NC]
RewriteCond %{REQUEST_URI} !\.(jpe?g?|png|gif) [NC]
RewriteRule .* /maintenance.php [R=302,L]

However I want to add in a condition that if the visitor's IP address is my own, it will not redirect me to the maintenance page and that I would be able to see and test the site as if it was online. This part is normally also straightforward:

RewriteEngine on
RewriteCond %{REMOTE_ADDR} !^11\.111\.111\.111
RewriteCond %{REQUEST_URI} !/maintenance.php$ [NC]
RewriteCond %{REQUEST_URI} !\.(jpe?g?|png|gif) [NC]
RewriteRule .* /maintenance.php [R=302,L]

However, because my web server is behind a load balancer, REMOTE_ADDR is resolved to the internal IP address of the Load Balance server.

How can I alter this to look for the forwarded IP address? I know in PHP you can use $_SERVER['HTTP_X_FORWARDED_FOR'] to get the forwarded IP address. I've tried a few things in the .htaccess but no luck:

%{X_FORWARDED_FOR}
%{HTTP:X_FORWARDED_FOR}
%{HTTP_X_FORWARDED_FOR}

SOLUTION

I got the following to work:

%{HTTP:X-FORWARDED-FOR}