I start with 14 calories per pound. So for example, I weigh 177, so my baseline would be 2478 (177 x 14).
Then I figure 80% of that figure: 1982 (2478 x 0.8).
So for me, a 20% deficit is around somewhere between 1900 and 2000 calories.
Now, that's a starting point - a guideline. I know, from having done this for about 2 years now, that if I eat 1900 calories and work out 3x-4x a week, I will lose weight, but veeeeerrry slowly. Like less than 1lb a week. So I tweak a little. I started dropping 100 calories at a time over a period of about 4 weeks.
Now I know that if I eat between 1600 and 1700 calories a day, get at least 30%-35% of that in protein, and am consistent with my exercise, I will lose close to 2 lbs a week.
The trick for me is sticking to both the calorie level and the exercise.
But as you can see, it wasn't just a straight calculation. It was starting with a guideline and tweaking and fixing and figuring it out as I went along.