Prepaid gratuities/tips favourable exchange rate?

Can someone please confirm my workings.

Gratuities rate for balcony room = 12.95 USD per person/per day

Party Size: 4

Cruise length: 17 nights (which equals 18 days)

Therefore total prepaid gratuities = 12.95 x 4 x 18 = 932.40 USD

My account on RC website shows 1,125.20 AUD

Official exchange rate 23rd July 2015: 1 AUD = 0.7371 USD

Therefore my amount payable now = $829.42 USD

So if I payed now I would save $102.98 USD.

Can anyone find fault with my logic as I wasn't expecting this to go in my favour.

I'm not 100% but don't they use 17 and not 18?

I assumed if they based it on nights, the gratuities package would say 12.95 per night instead of 12.95 per day.

Either way, even at 17, it is still a $51.18 USD saving.

I'm not 100% but don't they use 17 and not 18?

That's correct, they would pay based on number of nights.

Thanks for the responses guys. I'll guess I'll put that extra day into cash tips then.

Cheers