What cmpm said.
The QuickField is there for exactly this kind of purpose -- letting you quickly set part of the filenames for a sequence of screenshots without having to manually name each one. Note that you can combine the quickfield with other stuff like date and time, etc.
Now one thing to note is that Screenshot Captor is adding a 3-digit number (001, 002, etc.) part automatically, in order to come up with a unique filename (tip: It will put this uniquifying number where you have %num% if you specify that explicitly, or add it to the end if not). But one consequence of that is that if you take like: quick001,quick002,quick003, and then DELETE quick002 and take a new screenshot, the new one will be named quick002 (not quick004). That's because SC is looking for the first available free number.
Now if you don't want it to do that, you have 2 options. First, you could avoid using the number field completely and just use date and time.
Or, you can use the new %numinc% field in your filename, which will ALWAYS add a number, and it will always remember its value and increase one for each screenshot, even between reboots. To reset the number after a project, just go back to that file naming dialog and change the "Auto Incrementing Number" field back to 1.