I'm using my Canon 100mm f/2.8L macro handheld all the time together with a flash rig and it works wonders. Most of the time I have an Eg-S focusing screen installed as well which makes it even easier to get the focal plane where I want it. A couple of weeks ago I got the Kenko 1.4x teleconverter and started to use it together with my macro and with it I was able to take some even cooler shots.
As the results with the Kenko 1.4x were much better than I expected I decided to pick up their 3x teleconverter too as Mark Plonsky, one of my main sources of insipration, seems to be doing pretty well with it.
However, due to the three stop loss of light it is now considerably more difficult to manually focus using the viewfinder since unless lighting conditions are optimal it is very dark. Using liveview works really well but since many of my shots are handheld I usually steady the camera against, for instance, my shoulder and then liveview isn't really usable as it is too close to my eyes.
One thing I have considered to address this issue is to get a Brightscreen but reviewers seem to have mixed feelings about them. Do you have any suggestions on how I can manually focus with the three stop light loss without losing the flexibility of working handheld?