Having dealt with tax authorities in several countries, it's a recurring theme that they have no interest in coming down on you hard if you seem to be trying to do the right thing and make actual efforts at compliance, as they have their hands full putting actual effort into dealing with people actually trying to evade tax.

What I always do if in doubt is to attach a letter setting out my assumptions. I've outright had to tell the tax authorities I didn't know the real numbers one year, because I realised shortly before filing that I'd lost documentation in a move, and so a whole bunch of details were estimates. Even that was accepted without additional documentation.

Of course I'm sure there are countries that are worse.