Depending on how much your flat will cost.
Eg flat is 95k and add on stamp fees and etc we just conveniently round up to 100k.
If you are buying from HDB, you have to pay 2k downpayment first, which later can be offset from cpf.
Then, you have to deduct minimum 10% of the flat cost from your cpf, which in the case here is 10k. If the total of your cpfs do not have that amount, you have to pay the difference in cash.
If you wish to take loan from HDB, they will wipe your CPF account clean and offer you loan based on the remaining outstanding amount that the flat cost, into monthly installment deducting from your CPF. Of course the lesser the loan amount you take, the lesser you have to pay base on the tenure of your loan.
A comfortable purchase to me is one that I would not require to top up in cash for the monthly installment payment. Every month cpf is enough to cover and yet have some amount remaining.