It really depend on what you include in your mandays.
Here mandays includes exchange with clients.
Also 1 mandays doesn't mean 8 hours of work. because in one day you can't work for 8 hours on the same project.
For the number of mandays, every company have it's own recipe. here is the "wet finger" for the beginning, then rounded to the next Fibonacci number, then multiply by the estimate scale of the project.
for a navigation bar for example, with a good code base, it will be 0.5 for code, 0.5 for testing, 0.5 for clients exchange. so 1.5, next Fibonacci number is 2. size of the project is small aka 1.5 so result in 3 days.
Also notes that imho : going lower than 0.5 is too much details, we fail at estimation therefore better be safe, giving details will make your pricing more legit, best advice I can give is to pick little ideas from everyone to make your own recipe, try and experiment.
About efficiency, Pomodoro is nice but not really natural for me as you don't follow your own rhythm. I rather turn off every auto-update or notifications. I check them when I'm available and that my choice, not my notification's choice.
Make your own way, try and fail, that's healthy :)