As I understand it, the rate of interest, which is 12% per Real Year on White Giant should be divided by the number of game months per Real Year. As there are 4 game months per day, then 4 game months per 365 real days, gives us a total of 1460 game months per Real Year.
Therefore, 12% divided by 1460 game months, gives us an interest rate of approximately 0.008% (rounded up). Depending on what loan you have offered, you should earn 0.008% every month, which would eventually result in a 12% interest if the loan is available for 1460 game months (This equates to approximately 121.67 game years!)
Now, based on a loan of 100B, the interest will be 12B over a real year or 1460 game months. However, as I only want to give the loan for 1 game month only, the return will only be 1/1460th of 12B. 12B divided by 1460 = 8,219,178.08.
Scaling up, a 1T loan, which should earn 120B over a Real Game year, will earn you 82,191,780.82.
Now, going on the example provided by Jo:-
12% of 70Trillion is 8.4 Trillion;
8.4 Trillion/1460 is 5.75B (rounded up). This is close enough for me.
So, the equation to calculate a months worth of interest on a loan is:
((value of loan x 1.12)- value of the loan) / number of game months in a real year = value of interest earned during 1 game month.
Eureka!