a substance to promote... is unidiomatic.
the only context in which i can remember "a NOUN to VERB" is a context in which the NOUN is abstract in nature. for instance:
a way to produce goods
a reason to try harder
etc.
note that "substance" is a concrete item; it's not an abstraction like "way" or "reason".
new SD=1/n sum [30%Xi - 30%E(X)]^2,可以把30%提出来,所以第一个条件就够了