Net income breakup for self employed

If you're setting up as freelance, this is exactly the sort of question you should be asking your accountant. No one here can really answer for you as we have no idea of what expenses you're going to run through your business.

It's complicated. Very. 400/day should give you roughly the same income as 4000/month, but given that you have a lot more flexibility as to what to do with the money and have a lot more complex tax planning to do, you need to talk to an accountant. NOW.

Let's do a quick calculation:

400 day X 20 days x 11 months = 90K Eur approx

Then you have to deduct: tax free ceiling, social contributions, mortgage, and expenses that you might have as company car cost, etc.

That will be your taxable income, that is taxed at the same insane confiscatory rate as employees.

For 400 Eur daily, I would say is 3500 eur net, approx.

Ah, I forgot. From income above 70K, usually it is more advantageous to create your own company (SPRL), then get paid a low salary and the rest in dividends.
Talk to an accountant.

in addition to your other post on net salary some (free) advice from a long-term independant in Belgium:

Neocon's quick calculation is in general fine, but I would never count with 20 days income/month: you have to have in mind / your calcul also time for your own administration / your accounting / hours for client's acquisition (you need more than 1 client after year 1-2, otherwise you will not be accepted as independant) , preparation of offers and any other hours you cannot bill ...for us, it's coming closer to 15 days/month.

More important is to realize the following:
1) the social security has to be paid also in periods you have a low / no income (holidays, bad health etc.) - thus you need savings to cover those periods.
2) I was / we are running very well with the following formula for coverage:
any income was split into 1/3 social contributions/taxes etc.; 1/3 for for income and 1/3 into savings for less good times / holidays / unexpected higher contributions to taxes or social security as those are recalculated after your annual tax calculations (your tax bill is the base for your social contributions).

3) and please do not underestimate: if you are single worker within your company: if you work for 1 client, you will not be available to another ..or working more than the usual working hours / on weekends