Estes, I live about 13miles north of Lockhart, just off Texas 21.
There no such thing as a quickest way to get to Amarillo from anywhere in Texas, except for maybe Lubbock. I didnt even know anybody still lived up there anyway.
His best bet is to use Mapqwest.
Just off hand, I would say north on I35 to Ftworth then West on I-40 but I would have to look at a map to see what Highway goes from I-40 to Amarillo.