Can you still adjust your professional percentage within a cost category?

Yes, you can! :) I don't want to give you advice about percentages, but you can always adjust a percentage for future bookings.

There is no universal truth from the percentage for professional purposes (which is why I don't want to give advice about it), only estimates that you must be able to substantiate during an audit. So if you think 50% seems more accurate, do it!

There is no automatic check or anything like that. Only during an actual tax audit will they look at those percentages in detail, and then you will have the opportunity to provide an explanation as to why you chose 50% or 75% for something. It's not that they do this for everything; some are logically 100% or 75% or 50%. Only where they lack some context or it seems like a lot at first glance will you be asked a question.

Anyway, that's how it went for me a few years back ;-)

Yes, but if it's a one-off, maybe you shouldn't put it in the cost category?

You can also just adjust the percentage in the entry itself, then it's a one-off for that booking. The cost category itself will then remain with the original percentage.

Yes, that's true!