The
PoS difficulty can be found in several places.
1) In the header info/status of the block explorer.
2) With your local wallet, mouse-over the green check-mark in the bottom right--it will display PoW & PoS difficulties.
3) Within the wallet console, you can run several commands that will reveal info such as network difficulty. Type "help" for the command list and info.
4) There may also be a CLI parameter you can pass to the wallet to get this info, but of this I am less sure of this off the top of my head...
Inputs are the "containers" that hold "x" coins. A single address can have many inputs "within" it.
Please refer to the concept of inputs/outputs/addresses as it relates to blockchain--good info to brush up on.
The wallet evaluates each input as a separate "batch" of coins with a corresponding age.
Age for TZC PoS is explained in the whitepaper and in various supporting docs on the website, etc.
If you read up and still have questions, please feel free to come back and ask for clarification.
So generally speaking you are correct.
The # of coins you have (organized by input) is multiplied by the coin age.
This produces your coin weight.
Coin weight is loosely the PoS equivalent of hashrate in PoW mining. That should hopefully get you started and pointed in the right direction--good luck!

In other words, RTFM... Or in this case, the whitepaper.
But that's not quite what I am driving at here, rather, it's that what the aforementioned calculator asks for and what the wallet gives you appear to be different.
Surely you don't expect everyone who wishes to stake their TZC coins to have to read and understand the whitepaper first? That would be analogous to requiring every person who uses US dollars to first understand how the federal reserve works; while you could argue - and technically be right - that people /should/ understand the Fed, it's unrealistic to expect or demand it as a precondition for the use of their money.
So I guess what I am trying to emphasize here is that it should be easy for the average Joe who wants to use TZC as a general form of payment and storage of value to easily determine the interest rate, so to speak, from parking their coins in their wallet (ie - staking them).
I'm not asking you to explain it to me like I'm 5 - or even explain it all, really - but I am asking the wallet and/or coin devs to consider making the staking function of the wallet a bit more user-friendly.