The graph plots #theta# on the horizontal axis and #T/T_0# on the vertical axis. To use the graph to give you #theta#, we need to find the value of #T/T_0# when conditions have made the approximation go from valid to invalid.
#(T-T_0)/T_0 "must be smaller than " 0.01# for the approximation to be valid.
If #(T-T_0)/T_0# has been valid, but starts to get larger, it will still be valid until #(T-T_0)/T_0 = 0.01#. So, to answer your question, we first need to solve the expression #(T-T_0)/T_0 = 0.01# for #T/T_0#.
We have
#(T-T_0)/T_0 = 0.01#
and need to use some algebra on it.
#T/T_0 - T_0/T_0 = 0.01#
#T/T_0 - 1 = 0.01#
#T/T_0 = 0.01 + 1 = 1.01#
Now, look at your vertical axis and find where how high that value of #T/T_0# is. Go over to the curve at that height and then go straight down to the horizontal axis. Read the value of #theta#. I get #22^@#.
I hope this helps,
Steve