educated guess, an
A speculation based on past experience or knowledge, as in I'm not sure how much meat we need to feed twelve, but I'll make an educated guess and say six pounds. [Mid-1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.