It is o.k. to invest 20% of your income into bitcoin as long as you are investing from discretionary income.
it is a better practice to account for your expenses first, yet if you are sure that at least 20% of your income is going to be discretionary income, then there is no problem to invest 20% into bitcoin.
Also, if you have back up funds, such as an emergency funds and reserve funds, then those could save you if you make a mistake by accidentally investing too much into bitcoin in any given pay period.
I only invest 30% of my monthly income from each company salary, and this has only been implemented for the past few months even though I already know investing in bitcoin is good, but beginners may need to take courage in this attitude.
Yes, I always calculate the expenses first what is needed to be spent, after that the rest is to invest in bitcoin every month.
When it comes to reserve funds from the past, I have kept it because it is important for urgent needs, but now I am comfortable with the existence of reserve funds, which means that investment will be safe and will not be disturbed.