On monthly vs daily calculations - you should stick to monthly calculations when instalments are paid monthly otherwise, as you say, it gets messy.

Daily calculations should only be applied on variations, and you'd typically offset this from the last regular transaction.

Quote Originally Posted by Riebens View Post
IF I do monthly compounding for instance 12 % I get an APR of 1 + (0.12/12)12 for a 28 day month and for a 31 day month which works out nicely for a fixed debit order amount every month, while on a daily compounding I get for instance 1 + (.12/365) 28 and 1 + (.12/365)30 and 1 + (.12/365)31 which is not the same.
I think you've got a problem with the calculation you're using. The advertised rate needs to be the annual effective rate of interest, so unfortunately determining the monthly rate for compound interest is not as simple as "annual rate / 12".