What is the easiest way to figure out mixture ratios..?
It is 5 parts powder and 2 parts water.  Its an odd ratio..
So 50ml to 20ml is not quite enough.. That makes a total of 70ml,, I need like a total of 80ml...    

It may total 70ml but it probably won't actually mix-out to 70ml, especially when combining wet and dry. Is there any reason that you can't mix a little too much take what you need and call the waist a learning experience?

Cheop's Law sez that things
 

• Take Longer than expected
  
• Cost More than planned for
  
• Need More not Less of any given Resource
  

 Mix more than you think you'll need.  Unless you're talking about liquid gold cost is minimal, and it's better to have too much than too little of the resultant mix.

50ml - 20ml

60ml - 24ml

70ml - 28ml

80ml - 32ml

90ml - 36ml

100ml - 40ml

If you really want an exact 80 ml total, here's how I would calculate it.

Powder = 5
Water = 2

%Powder = 5/7 = 71.4%
%Water = 2/7 = 28.6%

For an exact 80 ml total:
80*0.714 = 57.12 ml of Powder
80-57.12 = 22.80 ml of Water

But you probably can't easily measure in hundredths of ml, so follow Roy's ratios and don't worry about wasting a little    

Finding mathematically correct volumes probably will not make the final volume you expect. As the dry absorbs the liquid the increase in volume may well be less than the volume of water that goes in. This even happens in liquid mixtures. 1/2 cup of alcohol and 1/2 cup of water does NOT equal 1 cup. Try it. There is enough intermolecular space in water to allow much of the alcohol to fit. Same thing happens in liquid/dry mixes.

Thanks guys,  I sort of had something figured out today on the lines of what Roy and jsweka says..