Handling a one-off starting data point?

One frequently encounters situations where you start with a number and periodically increase and decrease it by certain other numbers. For example, population - increase by births, decrease by deaths; savings account - increase by interest, decrease by withdrawals, etc. In all those cases, though, there is a beginning data point - starting population, intial deposit, etc.

How do you coceptualize this starting data point? Let's take the savings account, for example, and say the intial deposit is 100, and I otherwise have this table:

The report looks like this:

Obviously, somewhere in here I need to show a year-end number which will take the previous year ending balance (or, for 2016, the initial desposit) and add to it the total Activity for the year. So, in the table - where to I fit the initial deposit? And what do I do with it in the report? Do I add a column between 2016 and Activity and a row between Withdrawals and Total and stick there the intial deposit figure? And, finally, if the initial deposit is reflected in the table and report, is there a way to keep a row for total Activity for the year and another row for ending balance for that year (by that I mean, for example, that the 2016 column will show the same 10, -12, -2, but also a 98)?

I'm sorry if this sounds somewhat jumbled, but I asking not just about the mechanics of doing that in PBI (although I'm definitely interested in that, too), but more about the right way to think about it.

Thanks.