Wiktionary
guesstimation
n. 1 (context pejorative English) An estimation made without good justification and often biased. 2 estimation based on guesses, usually for expedience or because no better method is available.
Wikipedia
Guesstimation (game show)
Guesstimation is a BBC National Lottery game show that was broadcast on BBC One from 11 July to 29 August 2009. The programme was hosted by Nick Knowles and was based on the Dutch show Wat Schat je?.
Usage examples of "guesstimation".
Easing back a few inches while keeping the invisible sights lined up on it as best I could by guesstimation, I eased off the safety and squeezed the trigger.