How to calculate delay time (if you can't do it by feel...)
I could start off by saying that if you use delay or spin echo, then you should determine the time delay by how you feel it should sound. Just play with the controls until it sounds right.
But it doesn't hurt to get a little insight sometimes, and if you haven't tried this... well, you should...
You can easily calculate a range of interesting delay times if you know the tempo of the music.
And of course, the way music is made these days, the tempo is on the display right in front of you (And did you set that by feel? - whole other story there!)
So take that number and divide it into 60,000. 60,000 is a 'magic number' derived from 60 seconds in a minute and 1000 milliseconds in a second.
Say your tempo is 100 BPM (beats per minute). 60,000 divided by 100 is 600, so we have 600 milliseconds.
This is the base figure that will give a delay that is almost certainly too long.
But try dividing it by 2, or by 4. Try settings of 300 ms or 150 ms as your delay time. You will find that the delays fall exactly in time with the beats of your music.
Try dividing by 3, or divide by 3 then multiply by 2. So you have 200 ms and 400 ms. You now have a delay with a triplet feel.
You can try one-fifth, two-fifths or three fifths (even four fifths if you like), giving 120 ms, 240 ms etc.
More complex, but still related to the beat of the music.
Now I'm not saying that these timings are essential or compulsory. But they will be in time with the music, and the more complex divisions will be rhythmically interesting.
And when you have heard what these calculated delay times can do, your sense of 'feel' will be so much heightened.
Of course, some delay units and plug-ins allow you to enter tempo directly. But isn't it so much better when you understand how it works?