Here is a shot of the one that I use, I aim for around 10%. They are quite cheap on Ebay, here is an example:
https://www.ebay.com.au/itm/PRO-4-P...167571&hash=item1c9641ae2d:g:XIMAAOSwh1hZ6WY4
https://www.ebay.com.au/itm/PRO-4-P...167571&hash=item1c9641ae2d:g:XIMAAOSwh1hZ6WY4