Control chart boundaries for the proportion of words incorrectly typed by a typist per hour

Question

This a problem presented in Hoel's Probability Book Chapter 3. Suppose you wish to construct a control chart for the proportion of words incorrectly typed by a typist per hour. If she typed 1200 words an hour, on the average, for 6 hours a day, for 10 days, and she mistyped 360 words in that total period of time, what two numbers would you use for boundaries for the control chart? My solution, which I suspect is wrong, is: On average $1200$ words are type per hour, on an average of $6$ hours per day, for 10 days. Incorrectly typed words are 360 in that period of time. Every day 7200 words are type in 6 hours. Every 10 days 72,000 words are type. Let $p=\frac{7164}{7200}=0.995$ the percentage of total word typed, and $q=\frac{36}{7200}=0.005$ percentage of incorrectly words typed. Upper and lower chart limits are as follows: $$p+3\sqrt{\frac{pq}{n}}=0.995+3\sqrt{\frac{0.995\times0.005}{72000}}=0.9957 $$ $$ p-3\sqrt{\frac{pq}{n}}=0.995-3\sqrt{\frac{0.995\times0.005}{72000}}=0.9942$$