Traditional goal setting simply assumes a binary outcome for goal evaluation. This binary judgment does not consider a user’s effort, which may demotivate the user. This work explores the possibility of mitigating this negative impact with a slight modification on the goal evaluation criterion, by introducing a ‘margin’ that is widely used for quality control in the manufacturing fields. A margin represents a range near the goal where the user’s outcome will be regarded as ‘good enough’ even if the user fails to reach it. We explore users’ perceptions and behaviors through a large-scale survey study and a small-scale field experiment using a coaching system to promote physical activity. Our results provide positive evidence on the margin, such as lowering the burden of goal achievement and increasing motivation to make attempts. We discuss practical design implications on margin-enabled goal setting and evaluation for behavioral change support systems.