question- journey record

if thursday and friday i have class at 1pm, i leave my house at 12.25 and 12.30

my rage T of possible earliest and latest is 12.00-12.40 pm     40 mins

my data will be Thursday  T 12.25        25/40    = 62.5%
                   for Friday      T 12.30        30/40    = 75%

but for Saturday, class start at 9.00 and i leave my house at 8.35    

should my rage T of possible earliest and latest be 8.00-12.40
or
i have to make another rage T for Saturday, 8.00-8.40  am
or
just write              Saturday T 8.35          35/40   = 87.5%  because anyway it's all within 40 mins rage.

???