That OEM “761 – BMW Individual Sun Protection” glass usually falls somewhere in the 35%–40% visible light transmission (VLT) range, so your guess of ~45% isn’t far off, but it may look slightly lighter than factory. Most factory privacy glass tends to appear darker from the outside because it’s dyed glass rather than film, so matching it perfectly with tint film can be a bit tricky. If the goal is a true OEM-style finish without too much contrast, 35% is generally the safest match for sedans like the E60.

The push toward 20% from your installer makes sense if you want a more aggressive look, but it will definitely be darker than factory and more noticeable against the front windows. Shops that specialize in OEM-style matching such as OC Tint Solutions, usually recommend testing film samples on glass before committing, since lighting conditions and interior color can change how the tint appears. You can also check: https://octintsolutions.com, to get a better idea of how different VLT levels translate visually on similar vehicles.

On an E60, a subtle gradient tends to look cleaner, especially compared to a Touring where heavier rear tint balances out the extra glass. If maintaining that factory aesthetic is the priority, sticking close to 35% (or even a high-quality ceramic film in that range) will likely give the most accurate result without going too dark.