Looking at the pull-back shot of the set-up, I think what I would have done is used a single flash head light placed at exactly the mid-back height of the subject, and aimed straight at the backdrop. I am familiar with the lights and power packs you have,since I own the same model of light heads. It looks like you're using an M90 with barn doors + a round reflector camera left AND the light from an umbrella camera right to make the backdrop white.
Being familiar with your brand of power pack, I think I might have gone with fewer watt-seconds aimed at white background--as noted, the backdrop has too much light being applied to it...it's already white, so it does not need too much light to make it amply white. A single flash head at 100 to 200 watt-seconds, aimed from right behind the subject, will be enough to make the background pure white, without causing any wrap-around or "blowback". From there, it's up to the Chief Lighting Technician to figure out the best main light position.
In a room with such a low ceiling, rather than a table, I would prefer to have my portrait "sitters" actually "sitting"...on a posing stool. That would allow you to position the main light high enough to get the kind of facial modeling and shadows you want.
A low ceiling like that makes getting a 45 to 48 inch umbrella main light elevated enough to get the kind of effect you might wish to be able to achieve.
Using a smaller umbrella, like a 30 or 32-incher would be an option, as would be cutting some black paper and taping it to the front of the umbrella to create a circle mask or a rectangle mask, to make the light smaller. It's tricky with ceilings so,so low, so lowering the subject,and moving the subject much closer to the camera, and farther away from the background, would both be worth the effort.
I've used Speedotron Brown Line stuff for a long time (20+ years), and am aware that the 800 watt-second power pack you guys have is a bit too much in such a small room. The usual solution with a white background material would be to reverse the main and background lights, and use the 3 and 4 outlets, the "B" channel outlets, in Asymmetrical mode, and pop the backdrop with the lower-channel lights, and use the higher-power 1 or 2 outlets. However, there is another way to set the studio up...
One old "SECRET" to using that 800 watt-second pack with 4 outlets in such a confined space is to use the 1 and 2 outlets for the background using a GRAY FABRIC, and bring it up to white, and use the 3 and 4 outlets for main/fill or just main lighting. In a really confined shooting area, I have had fantastic success by using Asymmetrical Power, the HIGHER-powered two outlets, to light dark backdrops and "elevate" gray to white, and using less light on the foreground. Specifically, set the power pack to Asymmetrical, and aim lights 1 and 2 at a gray fabric background. Each light will be at 240 watt-seconds. Then, lights in outlets 3 and 4 will deliver 60 watt-seconds each--which with M90 heads in Lastolite Umbrellas, is a LOT more light than it might seem. Your gray will be a beautiful white background, and the main light will be about right. One light will be enough in that room to light the face with a nice 3:1 or so ratio, and you might even just aim the second light off a wall to bounce back and make a slight rim light, or use the M90 and barndoors and a diffuser to make a soft,gentle separation light. Or, just point it out the window and throw away the excess light.
That process, using LESS main light, and more light on a gray background to elevate gray to pure white is called "key-shifting", and I learned how to do it from a Dean Collins article I clipped out and still have in a clip-book portfolio. Here's a link to a shot I did on gray seamless in 2006, using 4 lights. Two on the backdrop in outlets 1 and 2, and a hair light and a main light in the lower-powered outlets 3 and 4, using the Asymmetrical option your power pack has. With such a low ceiling, and your power pack and lights, I think I would work on gray,instead of a white background, as I did in the photo I link to here
_DSC4648_Justin_sRGB.jpg photo - Derrel photos at pbase.com