Small prints are nothing to be embarrassed about
8 drops total is probably in the right ball park. But rather than use a formulaic answer, I'd ask you how it feels when you're brushing.
Does it feel like you're having to work the brush to cover the area? Does it feel like all the sensitiser has dried but you're still trying to brush out to the edges of your print? If so then add a bit more sensitiser.
On the other hand, do you find that you've covered your print area, but you've still got pools of sensitiser on the surface? Once you've covered the print area, do you still see wet trails off the bristle edges as you brush? Once you've covered the print area are you still getting puddles of sensitiser at the edges? If so then you've got too much sensitiser.
Regarding the ratio of ferric oxalate to pt/pd solution. These should always be equal. So if you want to increase from 4+4 then go to 5+5. And to decrease go to 3+3. If you want finer control then mix up a few prints worth of sensitiser (e.g. 10+10) and then count out 7, 8 or 9 drops of this mixed solution.
Hope this help.
Ian.