Retirement tax questions

...don't do it on your actual 2018 tax forms..leave that alone..make sure it is back the way you started back the way it was hen you filed...and saved that way

.....and then do a Save-As to some test file name. and only mess with that.

when you increase your box 12 amount, you must also reduce your box 1 value.  the box 1 value has the major effect.....the box 12 value for that particular entry is primarily an explanation for why box 1 is what it is.   Other  box 12 values can also affect box 1.  So if you increase your box 12 code D by $1,000, you then reduce box 1 by $1,000