Gram per watt devided by the days times 100 I think would be a good way of taking the rate of growth into account.
For example:
1 gram per watt, 65 days total you'd get:
1/65x100 = 1.538
1 gram per watt, 75 days total you'd get:
1/75x100 = 1.333
1 gram per watt, 90 days total you'd get...