formula for calculating weight to a desired value up to a specific date

Hi friends

I WANT to calculate the weight on 60th day = based on the weight per day after 45 days in cell H8 + the value of weight on 45day in cell G8

cell I8 value = 0.31*15+7.92= 12.61(not rounded)

like the above calculation i want for 75th day, 90th day,105day in increment of 15 days and should not cross the total days.

if final day falls in between 15 days increment calculate extra days of the previous increement and equal to total days calculate the weight on the lasy day.

weight also should not cross the value in the cell C8 that is final day weight should be value of the cell in C8

for the above logic i want to create a formula to drag and calculate

kindly help me with this to solve

Thanks in advance
green369