Yep, Peter, that's the 'standard' way of getting the white - just have to be precise with the trimming and/or re-touching.
It's not to bad on linear subjects such as lettering, but can be a bit tricky to align with complex shapes, such as some nose art.
Passing the sheet through the printer twice will, in theory, give double density ink, but the problem is precision registration. It might be possible to obtain straight, tight edges, but doubtful on the average printer. That's the reason I suggested using a darker shade as, even on a lighter background, due to the clear film being printed on, there is no base density to aid the colour saturation of the printed image.
One thing that is a must though, is a good sealant. I've just ordered a dedicated acrylic spray decal sealant, as I found that Johnson's Klear (Future) isn't the best medium to use. It works, but is prone to lifting in some instances when using, for example, Micro Sol. It is slightly better after around three coats, allowing plenty of hardening time between coats, but still not really a satisfactory coating.