Calculating Efficiency manually using metric system

To start with, I'd figure out the yield as a percentage. I find this number way more useful, especially as maltsters generally provide a yield as a percentage on their analysis sheets. To do this, divide the ppg of the grain by 46 (sucrose ppg). That gives you the percentage of the weight of the grain that will become extract.

e.g. 1.037 --> 37/46 = 80.4% or 0.804

Then multiply that by the weight of the grain to find out how much of the grain should become extract.

e.g. 5.5kg x .804 = 4.42kg extract

Do that for each grain, then add all the extract quantities together.

((37/46)x5.5)+((35/46)x0.5)+((35/46)x0.3) = total extract

Divide the total extract by the total wort weight (volume is close enough). That gives you your maximum gravity in Plato. In this case it's about 20 plato (20% extract by weight).

Now all you do is divide your actual gravity in plato by that number.

e.g. actual gravity 16 plato 16/20=80% efficiency

This may seem a bit foreign coming from a classic homebrew background, but after a little playing around, you will probably find that using plato makes a lot more sense if you want to understand the maths.

Hope that helps, feel free to ask for clarification.