It’s been a while since I’ve done any scanning in-game so I can’t remember if CCP has made any changes pertaining to signature signal strength. Also I’m sure someone will probably correct me if I’m wrong on this.
The way I remember it for faster scanning, Hacking sites would show group type at 25% signal strength so if you’re not looking for those then you can just select ‘ignore’ and delete that signature entry from the scan result list.
After 25% signal strength, any signature listed as ‘Unknown’ in the scan result will then be a wormhole or combat site. Scanning those signatures to 75% signal strength would show their group type so if you’re only looking for combat sites, then you can just ignore and delete the w-hole signatures from the results list.
Another option is to scan each site to 100%, bookmark it and then delete the signature from the scan result list, then when you leave and come back later, you can launch a single probe at small scan radius and position it on each bookmark to see if the site is still there or not. If the site is still there, just ignore / delete it from the scan result list. If the site is no longer there, then you just delete the bookmark.
After checking all bookmarks, you then do a large scan of the system to check for any new signatures that may have spawned. If some new sigs show up, just do the same to those that you did with the other sites earlier.