Question event function between FixedUpdates

I have 0.022222ms fixed time stamp.
Sometimes I make a lot of physics operations, which takes 1ms for calculations.
Would be good to performs these actions in between fixed updates. In profiler I can clearly see free space just begging to be used.

Question is: how can I do something perfectly between FixedUpdates?



EDIT: I think I did something similar by adding this into coroutine:

yield return new WaitForFixedUpdate();

yield return new WaitForEndOfFrame();

yield return new WaitForEndOfFrame();

But I'm not sure it's exactly correct. In case it's important - game is for VR so target FPS is 90. Twice the amount of fixed time stamp