Here are the percentages on reflectivity according to the Indoor Grower's Bible...

Foylon 94-95
Reflective Mylar 90-95
Flat White Paint 85-93
Semi-Gloss White 75-80
Flat Yellow 70-80
Aluminum Foil 70-75

The real point though is how many lumens are getting reflected. The material most be close to plants to make much difference. Light diminishes exponentially by distance. 100% at 1 foot is about 25% at 2 feet and under 10% at 3 feet. So if the light has far to travel to your reflective surface and back to the plant, it becomes ineffective. Bright to your eye does not translate to bright to the plant. Useful lumens is the key.