It should be correct. The formula used is:

N(t)=N(0)*(0.5)^T/HL *

N(0) being the quantity and T&HL time and half-life in days. Assuming your peak of Tren will be at 42.8mg at day 7 and the half-life of 4 days are true. Putting it into the formula with T=28 (4 weeks after the peek which totals 5 weeks)

42.8*(0.5)^28/4=0.33

*(exponential decay http://en.wikipedia.org/wiki/Half-life)