The odds of irreparably bungling a part or assembly can be stated as 1:SCT(1+F)/P, where S = volumetric size of the part/assembly, C is the cost, T is the time invested so far, F is a measure of your fatigue, and P is the percentage left to completion.
F is the only factor that can be controlled and varies by person and situation, being a scale of 100 and maxing X minutes after starting work. Personal observation has shown X to range from 10 minutes to 3 hours, with 70 minutes being average. Indications of Fmax can include: searching five minutes for the part/tool that is on the worktable in plain sight ready to be used, reheating your cold coffee for the third time before taking a sip, not being able to find a pair of safety glasses despite having several kept around the shop, and so forth. It can also be measured by the rude word ratio: when the ratio of rude words to normal speech approaches 1:1, Fmax is near. If the ratio over any 2 minute period reaches 5:1 (such as after dropping a bucking bar on your sandal-clad foot), Fmax has been reached.