I am presuming you are in the child benefit question section here ?? - not sure where step 1 is detailed (i don;t have acces to hmrc online submision version easily so it may be there ?)
I don't understand if in this amount will be included also one-off capital gains, this is not mentioned in the list of examples and there's another dedicated section later for it.
I can confrim that capital gain are ignored here as a capital gain income is ignored when it come to working out whether there is the adverse tax charge ref income for capital gains purposes.
Note if you have any doubts here and know that you would have to pay the child benefit tax charge due to having higher income and a claim has ben made i would recommend if you have the slightest doubts entering the figures as if income was more than 50k - checking that 9inclusion affect calculation generated before submitting if you are doing online submision - the system should self check before submisison and confrim if your income is over 50k - that is potentially more reliable than you trying to work out how the 50k is arrived at !! - if there is an issue you don't undertand (unexpected liability) you have the opportunity to check with hmrc or post back here.
Once you are happy there is no tax charge simply change that section back to No before submision
for paper return you cant self check but if you do enter and income is less than 50k that wont adversely affect your tax calc like it would do if you get tyour calcs wrong and miss of by mistake
Unfortuantely its ahighly technical calculation that should have a full list of yes no items with notes - i dont think that useful complete list exists ??
Note any pension payments made direct by you or deducted from your net pay (ie with hmrc adding aetra relief directly into your pension pot) - reduce your income ref this child benefit question hmrc seem to have conveniently for them not mention this reduction in that section you quoted.