How high can I tune my network stack buffer size to increase network performance due to long distance server latency?

I followed this guide: https://www.cyberciti.biz/faq/linux-tcp-tuning/

I added the following values into my /etc/sysctl.conf file:

net.core.wmem_max=12582912
net.core.rmem_max=12582912
net.ipv4.tcp_rmem= 10240 87380 12582912
net.ipv4.tcp_wmem= 10240 87380 12582912
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_timestamps = 1
net.ipv4.tcp_sack = 1
net.ipv4.tcp_no_metrics_save = 1
net.core.netdev_max_backlog = 5000

The server I'm using is just for OpenVPN usage and has 1GB of free memory. The guide currently sets the buffer size to 12MB, but I want to MAXIMUM I can set.

How can I adjust these settings to the MAX? My main goal is to increase performance over an OpenVPN connection that has a latency of 200ms-300ms.