Let Photoshop Do The Math When Copying/Pasting
If you’ve selected something within your Photoshop document and you want to copy-and-paste that item into its own separate document, don’t worry about typing the Width and Height into Photoshop’s New document dialog. Photoshop automatically figures that you’re going to paste that image into a new document, so when you open the New dialog, the exact size of your copied selection has already been entered for you, so just click OK, and then paste your image inside—it’ll be a perfect fit.