So it turns out I've been doing the maths wrong. Very wrong. It's much easier to calculate.
TOTAL BTC spent on FeeShares is the BTC Value of the ShareCoins you used.

((Total BTC spent on FeeShares x 100,000) x 1000) / Number of FeeShares Owned = Daily Transaction volume SharexCoin needs to earn your investment back in one day.

(((Total BTC spent on FeeShares x 100,000) x 1000) / 30) / Number of FeeShares Owned = Daily Transaction volume SharexCoin needs to earn your investment back in one month.

(((Total BTC spent on FeeShares x 100,000) x 1000) / 365) / Number of FeeShares Owned = Daily Transaction volume SharexCoin needs to earn your investment back in one year.


So if you purchase 1 FeeShare for 0.07 BTC (ShareCoins valued at 350 satoshi each).

7,000,000 BTC Daily Transaction volume SharexCoin needs to earn YOU 0.07 BTC back in one day
233333.33 BTC Daily Transaction volume SharexCoin needs to earn YOU 0.07 BTC back in one month
19178.08 BTC Daily Transaction volume SharexCoin needs to earn YOU 0.07 BTC back in one year