So if I'm understanding correctly, you did one series of four exposures at f:11, four at f:8, and four at 5.6. My suspicion is that you will discover that all four exposures at f:11 will be essentially identical, as will all four at f:8, and all four at 5.6. Ambient light at this strength will make no significant contribution to the exposure at 1/30 sec. with this amount of flash power.
If the flash exposure for your flash unit is f:8 at the chosen distance and film speed (based on the guide number, I assume), then I would do a series all at f:8 from the fastest speed to the slowest speed on your shutter. Look at the contact sheet, and note at what shutter speed you can detect a difference in the exposure based on the contribution of the ambient light.
In average room lighting, an ambient exposure with no flash at EI 400 should be around 1/30 sec. at f:2.0, so if you're shooting at EI 100, that would be 1/8 sec. at f:2.0 or 2 sec. at f:8 (plus reciprocity, if you're using a traditional film). Under these conditions, you will probably not notice any significant contribution from ambient light until you're at around 1/2 sec.