Welcome to PA, emmataylor.
The AMO standard for an arrow is, it will be 28" long with a 125 gr head. The minor adjustments you are talking about is, for each inch longer than 28" the effective spine value will decrease by 5# under the given spine and increase by 5# per inch shorter than 28".
With the 125gr head, for each 25gr over 125gr the effective spine weight will decrease 5# and each 25gr under the 125gr will increase the spine by 5#. So you can start with a shaft with a given spine, add or subtract to the shaft length and/or head weight to find the arrow that you can shoot well. You and how you shoot are also part of the equation.
I hope I haven't confused you.