What you should do is ask CPF to transfer your entire balance to a local bank account. Then from there, you can remit whatever sums as and when you nedd it...and especially observing the exchange rates.
It is also better to make that trip to ICA & renounce there and then with all your original documents.
The process is that ICA will issue you a letter of certifying that you have renounced and with that letter only then you can approach CPF for processing your withdrawal of the monies.
You will also need to get your local bank to certify that your account is indeed genuine before CPF process your application. Any Singtel shares you have in your CPF shares accoun, you should starting selling them off now. I did not even know I had them and had to endure more documentation processing as a result.
Expect up to 6 weeks for completion.