I paid khums on 25th of November which is also the day I receive my salary.
I combined the total amount in both my regular account and my savings account, and paid 20% of that amount
I noted down how much I had left and how much I paid in Khums
In this case I had about 1200£ left after I've paid my khums

Now next year, do I just do the same thing by gathering the total amount, but instead minus the 1200£ and then 20% off the total amount?
Even if I used much of the 1200£ during the December month (expenses etc), I can still deduct 1200£ next year correct?

by

Yes so next year if you have £1400 in total (after doing all calculations and deducing deductible expenditure), you only pay khums on £200 which is £40