When testing proxies make sure your Antivirus is turned off, or at least the Network protection part. Otherwise it might block some proxies and proxy judges. Some AV's are really bad at that like Malwarebytes, Avast, ESet, Etc.
You might want to also increase the timeout setting on the proxy tester, if its set to low many proxies might fail. Since Gscrapper, Scrapebox, etc are all multi threaded you should not see a big impact in performance even with a higher timeout setting of 45 to 60 seconds.
Poll for new proxies at least every 5 minutes. Google is pretty quick to block proxies with to many searches but we are always adding new ones so if you poll the proxylist often enough you will always have working proxies.
If you are using Scrapebox, after your first testing of the list we suggest you do a "Retest Failed Proxies" You should then see a increase in working proxies.