Teach the child what it means when the job is, say, 10%, 50%, 80% or almost 100% done. And then teach the child where, when, how and to which percent completion to do various tasks.
Regarding this: "I wouldn't be wanting everything to be done as well as it can possibly be, but for everything to be fully completed":
Do not teach the child to do all tasks to near 100% completion. First of all, that's impossible in real life. And second, that’s the message that encourages perfectionism and anxiety. There is plenty of research done on this topic, please see below for a few examples.
In my personal experience, I teach my kids (and myself) to be flexible in the degree of task completion. I do not fully complete all tasks, on purpose, and not because of laziness. I point out such examples to the kids, and explain why. I teach them to be flexible about task completion. This patient work on purposeful partial completion bears fruit, in that the children are growing up with a more healthy (lower) level of perfectionism than they would have done without this intervention.
A side note: I am now less of a perfectionist myself thanks to this process.
Another side note: of course, some tasks must be done to 100% completion - don't get me wrong. But relax a bit about those dishes and that lawn patch! :)
The findings suggest that parent perfectionism and overcontrol, together, may represent a specific pathway of risk for the development of anxiety disorders in children.
Affrunti, N.W., Woodruff-Borden, J. Parental Perfectionism and Overcontrol: Examining Mechanisms in the Development of Child Anxiety. J Abnorm Child Psychol 43, 517–529 (2015). https://doi.org/10.1007/s10802-014-9914-5 : https://link.springer.com/article/10.1007/s10802-014-9914-5
Findings from these studies are beginning to implicate perfectionism as an underlying process that may contribute broadly to the development of anxiety and depression in a pediatric population.
Affrunti NW, Woodruff-Borden J. Perfectionism in pediatric anxiety and depressive disorders. Clin Child Fam Psychol Rev. 2014 Sep;17(3):299-317. doi: 10.1007/s10567-014-0164-4. PMID: 24481881 : https://pubmed.ncbi.nlm.nih.gov/24481881/
The anxious rearing model of perfectionism development proposes that children develop perfectionism in response to parental worry about their children being imperfect and parental behaviors such as overprotection from mistakes and focus on the negative consequences of mistakes. [...] Results support the perfectionistic rearing model and parental perfectionistic behaviors' impact on children's observed and self-reported SOP and task performance.
Mitchell JH, Broeren S, Newall C, Hudson JL. An experimental manipulation of maternal perfectionistic anxious rearing behaviors with anxious and non-anxious children. J Exp Child Psychol. 2013 Sep;116(1):1-18. doi: 10.1016/j.jecp.2012.12.006. Epub 2013 Feb 15. PMID: 23419410 : https://pubmed.ncbi.nlm.nih.gov/23419410/