No need to do math. All you really have to do is split the mic track at the very first transient. The length of the empty clip on the left there is your gap.
Or just draw a selection range between the first two transients and the tooltip will probably give you that number directly.
Or, don’t worry about the number, zoom in and just align the mic track to DI track manually by slipping the audio.
Why would anyone ever do manual math for that when the timeline itself is a calculator? 
Why delay the track when you can just slip the audio and line it up?
You’re over-thinking it all.