Why do you want to mix powder, I could understand it if you were wanting to try out some different developers but, I dunno, I've used loads of developers and never had to mix powder.
I have some Ilford microphen, its powder, dunno what century I'll get round to mixing some when there are so many in easy liquid form around..
I think he's mixing "stock" developer, that's full strength, you'd still have to dilute the "stock" to suit the film and the developing time you chose, as and when you soup a roll.
With a one 120 roll patterson tank, you need 500ml of mixed developer and 500ml of mixed fixer.
So I'm using errrr.......Ilford FP4 + 125, and errr.....Ilfosol s liquid developer.
In the massive dev chart it says -
FP4+ Ilfosol S 1+9 125 6.5 20C
1+9 = 1 part developer to 9 parts water, so thats about 40ml of developer in 460ml of water @ 20 degrees for 6 1/2 minutes.
Fixer is 1+4 I think, depends on the brand, so thats 125ml of fixer in 375ml of water @ 20 degrees for 3-4 minutes.
So with film loaded in the tank, add you're 1+9 mix, invert 4 times at the end of every minute for 6 1/2 minutes, gently and tap the side of the tank.
Empty the developer out and flush with running water for 10 mins @ 20 deg, or use a stop bath.
Add you're 1+4 fixer, invert every minute for 3-4 minutes.
The fixer dissolves all of the undeveloped silver salts from the film, leaving the metallic silver that was produced by the developer.
Empty fixer out, and flush with running water for 10 minutes + whetting agent or photoflow or ilfotol or whatever, you are basically washing away all the crap and helping the film to dry without watermarks or streaks.
Personally, I don't use stop or whetting agent, but I think my water may be good for this, all that sheep p*ss in it off the Pennines.....