Re: paint thinning
Typically % reduction is for After you mix the paint ( paint or paint plus hardener ). So you have a gallon of paint you would add 1 qt of reducer..or 25% reduction by Volume of Mixed product.
Mixing by "part" is mixing equal parts of A,B and/or C together in equal measure ( 3:1:1 would be like 3 Qts paint 1 Qt Cat and 1 Qt reducer ).
Each Paint system is different.. What paint system are you talking about ( unless this is just a general question ) .
If you really wanted to get nuts you should be using a Zahn or other type of viscosity measuring control

. If doing multiple coats of reduced paint at one time..you will loose the reducer right out of the mix between coats..meaning your paint will become thicker from coat to coat. Re-reduction would be needed in this event..
YD.