Retirement tax questions

So if I haven't taken any distributions in previous years and just took the full withdrawal as one lump sum disbursement, I would essentially NOT subtract anything from the basis, correct? I'd only subtract disbursements if I had previously taken some prior to the most recent? In other words if the disbursement was $2600 and I find out that I contributed $1000, and I made no disbursements other than the full amount of the 2600, my basis would be $1000? Thanks a bunch!