Recently I've been shooting a ton of full length shots of all kinds on white and gray seamless. The shots have been turning out really great but I have some issues with the shadows cast on the paper underneath the subject and need some pointers to improve my technique.
For the sake of simplicity... let's say that I have a model standing on white seamless. My main light is positioned roughly 45 degrees camera left and about 30 degrees up from the lens's axis. I'm metering f/11 on the upper torso and the falloff gives me readings of about f/9 or f/10 as I meter at the lower leg or the shoe. There is a shadow cast behind the subject and to camera right by the main light that is very pleasing and gives it a sense of realism and dimension.
My background lights are rendering the white seamless behind the subject as pure white from top to bottom and left to right within the intended cropped subject area and as far forward as my gobos that hide the lights and prevent flare and halos. The gobos are placed about 6 feet from the background and the subject is standing just forward of that.
The seamless that is forward of the gobos and under the subject is not pure white and the shadows cast fall on this area which is approximately 1 to 1.3 stops under pure white. Due to the falloff there is also a gradient within the shadow and without using some advanced Photoshop techniques and taking a bit more time to process each image it's next to impossible to differentiate the shadow and gradient from the underexposed seamless.
I hope I haven't lost you yet 
I know many ways within Photoshop to mask, create gradients and simulate shadows to paint back in a shadow but I'm not thrilled with the time it takes and always seem to feel it doesn't accurately represent the lighting as well as the natural shadow created. Using the Dodge tool with the range set to Highlights works to a degree but since you can't fine tune the actual highlight range it always seems to destroy the outer edge of the shadow and again can't differentiate the shadow gradient from the underexposed seamless.
If I add secondary lighting specific to the floor it changes the appearance of the image and directionality of the shadows, and ultimately it still doesn't do what's needed.
So the question is.... how do those that do this type of work deal with the problem? Pouring light on the floor is obviously not the answer. I don't mind a bit of processing but most of the advanced techniques I know and have learned just don't accomplish this very well.
I welcome suggestions but please make those suggestions based on experience in this area or a working knowledge of this type of lighting. I mean no disrespect but I've heard all the "try this" and "try that" from many that are either just Photoshop guys or from those that guess with no experience. I want to tackle this as a lighting problem first and then as a post processing one.
The one time I did need to shoot in the style you described it was in a huge warehouse/studio so it wasn't too tricky.


