Filing Form 4868 without designating a definite payment?
can I file form 4868 without designating a payment if the tax return is still being processed and no balance due has been determined, yet?
can I file form 4868 without designating a payment if the tax return is still being processed and no balance due has been determined, yet?

The Form 4868 is not an extension on paying taxes. It is only an extension on date to file. You should pay a reasonable amount if he is going to owe money. and only the tax preparer can calculate the reasonable amount.
the problem is I haven't completed the client's return. The figure might change with time, so I donot want the clients to pay while the tax return is incomplete.

I usually have them pay about 1,000 to 2,000 more than what I expect them to owe to have them covered. Then if you get done with return soon, they can e-file and still get back overpay by early May.