Is this what you are trying to do? (If this is not what you were trying to do - I did not understand your question - sorry)
The attached are for illustrating - the size is not what you want -
5.75 inches finished is an "unusual" size - is that what the pattern calls for? or is that what you ended up with? From the picture that you have on the left - it appears to me that your "finished" 5.75 inches is including the seam allowances?
After establishing the size you want, I would draft the pattern and make templates.
This is a fairly good tutorial on how to make them -
How to make plastic templates