Sticking to schedule negatively impacting quality of work

I am trying to stick to the time I’ve allotted for a task. And I’m trying to be realistic about my scheduling. But with creative work (writing, designing), it sometimes takes a few tries to get to a satisfying outcome. (Note: It works both ways. I sometimes finish something in much than the allotted time!)
When I force myself to stick to the allotted time, I sometimes feel like I have to settle for something that’s not my best work. That it would be better if I gave myself more time–but that would require not doing something else on my schedule. Sometimes , I run out of time before I’ve thoroughly tested/checked my work and end up making errors. Should I just schedule in a bigger margin for error and plan to get less done?