When I need to do this, I divide the tempo of the song by 60,000. This gives the number of milliseconds that corresponds to a quarter note. Other values can be found from this value. For example, divide the result by 2 to get eighth notes, or divide the result by 3 to get eighth note triplets.

My prefered method is to insert the delay on a snare track, and tweak the knobs until I get the right rythmical pattern happening. When I get the right pattern happening, I move the delay to the track I 'really' wanted it to be on. Sometimes I'll use an aux for this...

Sure, you can do calculations, but that's not as fun, and often not as rewarding....

Cheers,

Kris