I'm not au fait with High Speed Thai and I'm hardly a linguist but I was taught the tone rules as I learned the different classes of consonanants and the long and short vowels when I studied Thai at Language Express in Ploenchit. I think that you'd need to master these rules too if you're serious about getting to grips with Thai. A middle class consonant, such as gor gai, with a long vowel gives a mid-tone. But when gor gai is accompanied by a short vowel then you get a low tone.
Enjoy your cold beer!