There isn’t a simple lumen formula for a lightbox because the effective light depends on diffusion, distance, positioning, and how much the box and subject reflect or absorb.

A practical way is to meter what you have now and work backward:

• Choose the aperture you want.
• Choose a handheld-safe shutter speed. A common rule is about 1 / focal length (or faster). For a 50mm lens, aim for roughly 1/50s to 1/100s.
• Check what shutter speed your camera currently suggests at that aperture.

Then compare the two. Each doubling of light gives you one stop faster shutter speed:

• 1/25s to 1/50s = 2× more light
• 1/25s to 1/100s = 4× more light

So if your current setup meters 1/25s and you want 1/100s, you need about 4× more light from a similar type of source.

You can also use EV calculations, but in practice metering your current setup is easier and more reliable.

That said, for product photography a tripod or monopod is often the better solution, since it avoids camera shake and makes careful composition easier.