First financial year not a full year – yearly costs fully or pro rata deductible?

I think you should calculate it as follows (note that there is not much left over, but still...):

1,200 euro fire insurance for a whole year (1/12) because only enabled for one month, starting on 1 December in your example * professional use percentage (say 20%) =

20 euros remaining for your accounts from the 1,200 euros of fire insurance. Not much, but still :)

That's how I would do it, curious to hear your feedback!Oops, I didn't do that during my start-up year (2021)... I did start in April, so it was only 3 months that were wrongly included?