Firstly, you need to understand that companies take time to calculate their tax price. Rest assured, most companies are literally in full rotation to get their financial reporting right and on time. No extra pressure is needed.
When is the best time to ask for a tax price?
In order to provide shareholders with a tax price, the issuers, i.e. the company that issued the shares you hold, need to finalise their annual report for the relevant financial year.
This process usually starts immediately after the end of the financial year, which may or may not be the calendar year, and attempts to be completed within 3 months, i.e. by the beginning of April if the financial year is the calendar year. This process means that the company may - by design - not be able to provide you with a provisional tax price when you file your personal tax statement.
What options do I have if I do not receive the tax price?
You can always submit the last definitive tax price or, if there is no definitive tax price, the preliminiary tax price provided by the company.
Normally, the company will communicate the provisional or final tax rate as soon as it is available. The best thing to do is to remain calm and submit the last tax price communicated to you.