Humm many things here.
I don't really want to start a journal yet but i may continue to share my input here.
My computer is dying and crashes every 30 minutes! So i need a system that isn't too much time consuming until i get my new beast (AMD3500+ w. SLI Nvidia and 30" LCD screen/TV and wall projector, oh yeah baby!)
For the rates, i took the rates from Oanda, where they show 2 rates (bid and ask). Like AUD rate are 5.2-5.57 and JPY is -0.2-0.14.
For the rate differential, i computed that way. Long AUD/JPY = bid AUD(5.2) - ask JPY (0.14). This way i found the interest rate spread of Oanda. For exemple, you long AUD/JPY you get a rate of 5.06, but if you short it's now a rate of -5.77 (a 0.71% net spread).
On average there's almost a 1% rate spread for every pair, so Oanda makes lot of money that way!
I then entered all pairs (with position side) that show a significant positive rate differential. There are no other available. So the goal is to balance exposure using only those pairs.
The little "hedging" pairs are those with little interest payments at worst. Like short CHF/JPY is almost essential to reduce some JPY short exposure, and you don't pay too much for this hedge.
Anyway my calculations seems pretty much on par with realty, i receive the same daily amount that i computed +/-1cent.
My system leaves room to gambling as i try to balance my exposure for the currency i think will be strong, in hope to make some extra FX gains or at least offset some losses. Let's say i'm more trying to build a heavily leveraged portfolio.
If i think CAD will be weak, i hedge long UAD/CAD. Else i do nothing, the fun thing is that the PIP value grows as the CAD gets stronger.
Exotic currency takes too much capital and reduce significantly ROI to be worth it (cause of the 1:25 leverage), but at the same time it does reduce risk, and there usually no inter-continental exposure with these pairs which is the main thing i try to avoid in my system. (i.e. i want to have exposure say on EUR and GBP and JPY and AUD, but i don't want to be over-exposed on the European continent relative to Asia, i try to stay neutral.) I think it's a way to avoid correlation between positions.