I think that down payment on a lease can be deducted over the lease period but what if the lease is terminated early (over shorter period than the original period)

Your lease downpayment must be prorated over the length of the lease. Example: your lease is for 5 years, claim 20% of your down payment per year and add that to your "total lease charges paid".

For a three year lease you would 34%, 33% and 33% of your downpayment each year. If you cancelled your lease before it was up, you will have to make a further proration and divide that 34% by 12, then multiply it by the number of months you held the lease.