For shielding with 6 mths tbill, can choose any tbill where the SA deduction is before your 55th birthday and matures after your 55th birthday. So if start early, plenty of choice but need to choose those that lose 7 mths cpf interest (those where issue date is not too near beginning of mth and maturity date is not near end of month).
For short term bond, to keep possibility of loss lower, better to hold it as short as possible. Eg apply 1 week before 55th birthday and sell on birthday.
Eg if birthday is on 3 Jul, means you purchase in June (eg 26 Jun) and sell on 3 Jul. So lose CPF interest for June and Jul. 2 mths loss.
If the birthday is on 10 Jul, purchase on 3 Jul and sell on 10 Jul. So lose CPF interest in Jul only. 1 mth loss.
If the birthday is on 29 Jul, purchase on 22 Jul and sell on 29 Jul. The money may not be returned to SA in time in Jul and end up posted in Aug. So lose CPF interest for Jul and Aug. 2 mth loss.
So if short term bond lose 2 mths CPF interest (base on your birthday) , overall sure worse off than shielding with tbill. So should just shield with tbill.
But if short term bond lose 1 mth CPF interest, then can either shield by short term bond or tbill. You have a choice.