Calculate deviation% based on first available value

Whitewater100  Thank you for the solution, which works perfectly in your case - and is just what I need. 

My dataset is, unfortunately, not build up with a row per day, but with a date range. F. ex. a purchase price is valid from Oct. 1 2018-Dec. 31 2019. And theres a new prices valid from Jan 1. 2020-current date.

I have tried to convert the date range to a list of dates to have the same layout as your solution, but it wouldn't work due to the amount of data.

Any suggestions how to solve the adjust your solution considering my date ranges?

Thanks a lot in advance.