When I'm building large nested if statements, I like to use a text editor like Sublime that can do parenthesis matching.
You can copy white space into a formula successfully (and it makes it a lot easier to read)
For true nesting, I like to build recursively. So start with your simpliest condition and put in placeholders like
IF(IsBlank(ABCD),"True","False")
Then copy in the whole statement and replace the appropriate True/False:
IF(IsBlank(ABCD),"True",IF(IsBlank(ABCDE),"True","False") )
I've even been known to create a temp form to carefully test different parts of an expression and then collapse them together.
I've built nested statements several pages long to encode complex business rules into a single value for conditional formatting of views, but it takes patience and LOTS of care.
I also keep a seperate documentation page with a table of my calculated fields, the calculations and an explanation of what is going on.
Finally I like to copy the calculation into the description of the field so that I can easily copy in the expression on a new form (or if the calculation gets damaged)
Here's a simple example:
IF(IsBlank(Agreed Budget Estimate),"Planning",
IF(Actual Revenue>Agreed Budget Estimate,IF(Actual Revenue-Agreed Budget Estimate<2,"On Budget","Over Budget"),
IF(IsBlank(Actual Completion Date),
IF(Actual Revenue>(Agreed Budget Estimate*.95),"Near Budget",
IF(Planned Revenue>Agreed Budget Estimate,"Planned Over Budget",
IF(Planned Revenue=Agreed Budget Estimate,"On Budget",
"Under Budget"))),
IF(Actual Revenue=Agreed Budget Estimate,"On Budget","Under Budget"))))