Выражение встречается в следующем контексте:

Заранее спасибо

Почему не предположить, что в контексте "In conservative way we take into account just 75% of thread length" In conservative way означает по традиции, по привычке, по старинке?
Почему "С учетом запаса прочности"? контекст это не подтверждает.

"conservative
5.  A conservative estimate or guess is one in which you are cautious and estimate or guess a low amount which is probably less than the real amount.
* A conservative estimate of the bill, so far, is about £22,000.
* This guess is probably on the conservative side.
* The bequest is conservatively estimated at £30 million."

— Collins English Dictionary

So the author's giving a figure of 75%, but saying that in reality it could be more.