Does the II chord in the Phrygian II-i cadence count as tritone substitution?

1. How do you calculate tritone substitution?
2. What is a substitute dominant chord?
3. What is modal cadence?

How do you calculate tritone substitution?

The tritone substitution can be performed by exchanging a dominant seventh chord for another dominant seven chord which is a tritone away from it. For example, in the key of C major one can use D♭⁷ instead of G⁷. (D♭ is a tritone away from G).

What is a substitute dominant chord?

Any chord that can be preceded by a secondary dominant can also be preceded by a substitute dominant, which is a dominant chord that is one half step above the root of the destination chord. And this substitute dominant works because it shares some notes with the secondary dominant it is substituting for.

What is modal cadence?

Modal cadences are - necessarily - weaker than tonal ones (ie in major or harmonic minor) - but that should be seen as a positive. The whole attraction of modal music (at least for musicians in jazz or rock) is that it avoids the "perfect" cadences of key-based music.