Using the DSL Reports formula,
http://www.dslreports.com/faq/578
of ping (in mS, but should be a LARGE packet ping, this may be a small packet) multiplied by line speed (in Kbit/s - the K cancels the m) x 1.5 (fudge factor) divided by 8 (bits in a byte).
169 x 768 x 1.5 / 8 = 24336
Your RWIN is actually too small to achieve full speed on a path with a delay exceeding 100mS