It's probably T 15 No. 613 Chan miyaofa jing. This text is only extant in Chinese, and AFAIK it hasn't been translated into English. See A Study of the Meditation Methods in the DESM and Other Early Chinese Texts
by Ven. Dr. Yuanci.
* N.B. This paper first lists the text as T 15 No. 603
, which is incorrect, but later corrects this to T 15 No. 613
Wow my friend, this is EXACTLY the sort of response I was hoping for! Well, maybe not EXACTLY, haha. I was crossing many fingers for info on an easy to obtain, English translation of the sutra. This is still more than wonderful, thank you! EDIT: Afterthought, does anyone know how to get ahold of a Chinese copy???