Drilling Rig Allocation and Contract Renewals

Dear Friends:

I am trying to solve a problem in the attached spreadsheet. I have to allocate a Drilling ring in a specified year with a certain starting day rate. Every 4 years (period specified by the user), the contract will be renewed at a new day rate (the rate will be specified). Could you please help me automate these calculations? I have 25 years and up to 10 rigs will be introduced at different times. I used the MOD function but it is not giving me correct answer.

I have made it to work but it is not flexible, i.e. as soon as I change any of the values in Cell C1, C2 OR C3 my results are messed up.

Your prompt response will be very highly appreciated.

Thanks and regards,