Which cost category should I use for payments to, for example, GoForest, and what is the professional use percentage for this?

Hi!

I don't know GoForest (apart from what I've looked up), what exactly will they do for you? Is it training? Coaching?

Go Forest is a partner organisation of CO2logic and aims to inspire and motivate individuals and companies to act in a future-oriented and sustainable manner. Through a kind of subscription formula (e.g. €50/month), you can reduce the CO2 impact of your own company.

Hmmm, I don't dare give an answer to that...

That doesn't seem to apply here, I think it falls more into the 'donations' category :) And donations are not tax-deductible for a sole proprietor. (Sometimes they are deductible in personal income tax, but then only as a private person).