Having decided when you want to retire, you should next look at how you'll want to live.
Think of it as a permanent holiday, perhaps?

A rule of thumb might be to aim for an income of a half to two-thirds of your current one – adjusted for inflation and set to keep pace with it in retirement.

Consider what you'll be doing. Will you be taking more holidays or pursuing an expensive hobby? The chances are, you'll want to carry on living at the same standard as you do now.