Dear Karl.
You should be able to get good non transformed measurements with a V filter that are within 0.1 mag error.
This depends on the camera you use as well as the filter and how rigorous you are at taking your measurements.
What camera are you using?
How linear is its response?
How are you measuring the mag?
Are you properly reducing the imaged with flats, darks etc?
There is lots of info on the AAVSO site and other good info about filters here
http://reductionism.net.seanic.net/C...ternative.html
The doc is a bit heavy going but it explains well how the filters work and how colour changes your measurements.