The Permawash instructions to mix 3 oz. of Permawash into 1 gallon (128 oz) of water works out to 3 parts concentrate + 128 parts water = 131 parts of working strength Permawash bath.

So for any volume V of working strength Permawash bath wanted we need only measure (3/131)*V of concentrate and add (128/131)*V of water.

Example: We want 500ml of Permawash bath.

Measure (3/131)*500ml = 11.5ml of concentrate and add (128/131)*500ml = 488.5ml of water to make 500ml of Permawash bath.