So according to wikipedia "The reflectivity of bright aluminum foil is 88% while dull embossed foil is about 80%", this is far better than I thought. Markus Kaysers SolarSinter Project, appeared to use 4 lenses of about 6ft x 4ft total area, while I doubt these are focusing all available light, I am going to assume 100% efficiency for his design, as any increase of power for me will be a bonus.

So I will assume that Markus design is 120cm x 180cm for a total of 21600cm^2 of light collecting area. At 80% efficiency i will need 27000cm^2 of light collecting area, or more if i need to.

So I plan on using a 200cm x 200cm square reflector, and loosing 20% for the focusing mirror, so 40000cm2 - 20% = 32000cm2 light collecting area.

Assuming Markus design is running at 100% and my own is 80%, this should give me at least 18.5% more power.