Corporation tax rate

Hello,

I suppose that I have a long period, of 15 month period to 15 March 2018.
I normally perform 2 CT calculations – for the first 12 month and for the remaining 3.

My question is what CT rate I need to apply for the first 12 month? Given the fact that the financial year starts at 01.04.2017 and my first period starts at 01.01 2017.

I am sorry if I didn’t get correctly all the information from video.

Thank you