How do you plan on risks when working on the project detailed plan? I
assume you add to the plan vacation days for each resource (even
though they may not have requested leave at this point), you plan sick
leave days, time for meetings attendance and most probably 6.5 or 7
hours effective working time per day.
After you look at your project plan you should always ask yourself: "What could possibly go wrong?. Some people avoid that question because they believe they could solve problems when they come up.
A good idea would be to involve your team in this discussion and maybe have a look at reviews of previous projects, so you can see what happened to those.
When you have a list of all possible risks you need to assess which ones to worry about. A good way to do this is to give them scores for "how likely?"(0 to 5) and "how serious?"(0 to 5). After that you need to multiply the numbers. If the multiplied score goes into double figures than it is a risk that should be concerned about.
Ffor that kind of a risk you need to have a plan. There are two types of plan you can have for reducing risks: you can either make it less likely to happen or less serious if it does happen.
What customers are interested in is "Which of the risks have you not been able to handle?" So, when you are showing your risk plan to a customer, it's important for them to understand that:
a. You've looked at risks
b. You're on top of risks
c. They're still exposed to risks
Do you also add in buffer time for development in case the effort required is underestimated by the developers? If yes, isn't that too risky in a sense that it may delay the project deadline unnecessarily?
Now this is a bit tricky. It has to do more with an estimate. Let's assume that you need to estimate how many weeks it would take to make a feature. Keep in mind 3 numbers: minimum amount, average (based on past experience) and maximum amount.
Let's assume you would have: minimum 20 weeks, average 30 weeks and maximum 60 weeks. Going halfway between the average and the worst estimate would be the best choice (30+60)/2= 45 weeks.
Most people start either with average or the average+10%. But adding 10% in most cases is just not enough. In our case we would have +3 weeks (30 +10%). 33 weeks would probably be not enough.