Best/Most accurate interval timer app??

I’m training for the Chicago marathon and I recently got a running coach who has included a couple weekly workouts that have distance intervals at certain speeds. I use Strava on my Apple Watch and was using Intervals Pro but today I noticed the distances are wildly off. For every “mile” I complete on the interval app Strava records about .85 which is quite the discrepancy. I’ve been making it work for now but I have shorter intervals (400m splits) coming up and I was just looking for something more accurate where I can include pace goals. Any recommendations?? TIA