Wrong Exchange Rate for Currency of little value

I'm trying to setup "Indonesian Rupiah (IDR)" as a base currency, however when I update the exchange rate is showing the wrong value for USD and EUR, etc. Instead of showing 0.00007633587786, it's showing just 0.00010000. This is throwing all the wrong price on the front end. I believe it has to do with the number of decimals and it just rounded up to 4 decimals. How can I change this so it works as it suppose to be giving me the right rate?

Another question is that, as you all know the global exchange rate is most likely not the local bank exchange rate. Is there a way to either feed data from other foreign exchange rate data, or for a more simplified solution, is it possible to hardcode adding certain value? For example, suppose according to global exchange rate that opencart uses, 1 USD is Rp. 13.000, I would like this to always add Rp. 500, so it will always uses the amount Rp. 13.500 for the calculation of the prices. And when the global changes to Rp. 13.200, it will use Rp. 13.700 as the exchange rate, etc. Is this possible?

Thanks. I hope someone would kindly give me a solution in the reply.

Chris.