Question about Focus Goal

While putting focus goal sub-categories into calendar, is it ok to add buffer time as one of the sub-category. I have chosen to focus on my work. Have 6 features to complete – each of which can be broken down into small components (analysis, design, coding,…). I am not completely sure how much each piece for each feature may take. It may be more or less depending on the complexity. So can I add buffer for each or a overall buffer ? Also, does my approach look correct ?