Hi Joe,
what t-test are you trying to calculate?
You should write what is the H0 distribution, whats is the critical value, what is the H1 distribution
You are doing the left tail. (one tail not two tails)
I prefer to calculate only the priori test power, in this case, you calculates the other avg(x) based on the "change" that you want the test to identify:
change=avg(x1)-avg(x2)
In the z-test the H1 also has the normal distribution, but in the t-test the H1 has the non-central t distribution,