50/50 Mixture

[icestorm81]

Iv recently had a big turbo package and water/meth kit installed on my car. The methanol is almost out. How do I measure the correct amounts to get the 50/50 mixture right. Is it one litre of each or do I go by weight? The container in the boot holds one gallon. Also can you use normal tap water that has been filtered using a brita water filter or is it better off buying bottled water. Any words of advice would be very welcome, many thanks.