Take this production setup as an example: http://i.imgur.com/BCz67OU.png

I want 4 solder per second, the recipe for solder is this:
(8 solder = 4 solder plate, 1 resin, 2 seconds in machine of 1 speed = 2 seconds)

Since I want 4 solder per second, that means it only needs 1 machine. (Correctly shown in image)

For that solder, I want 2 solder plate per second, the recipe for solder plate is this:
(11 solder = 4 tin, 7 lead, 7 seconds in machine of 1 speed = 7 seconds)

So I produce 11/7 (1.57..) solder plate per second per machine, we need 1 extra machine that won't be running all the time. (Correctly shown in image)

For that solder I also need 0.5 resin per second, since I only require 1 resin per 2 seconds.
I want 0.5 resin per second, the recipe for resin is this:
(1 resin = 1 raw wood, 1 second in machine of 0.5 speed = 2 seconds)

So I correctly produce 1 resin per 2 seconds per machine if I use the first tier assembling machine.

Yet it still shows me I need 2 machines, why is this?

Hello rodonies
there are no error in calculation, you can't build a 0.5 item is not realistic
and to produce 4 soldier you need wait 1 cycle of the soldier recipe and it produce 8 soldier.
set the minimal of this recipe
if you want the perfect ratio you must set block limit
Imgur

the algorithme:
compute the engredients for all recipes
compute the assembling machine
compute anothers value