I had the 1204 machined down to 8mm, and used a 32tooth pulley from robotdigg, and the reason i used a 16tooth on the stepper was so that i had a little more refined control when micro-stepping or doing a full step hence the 2:1 but that was just my personal preference, if your wanting 1:1 I would use 32tooth on all items as your after good tooth contact and you do not need to have a solid transition due to the smaller pulley
400 steps per rev (Igentis stock motors)
4mm lead per rev (your leadscrew)
=.01 resolution… to me that is PERFECT - you can pick any multiple of .01mm and be on a full-step…
Of course, you can be on any multiple of .005 now @Martin_Bleakley 
But given this info I will probably go for all 32T, since that means only 1 type of pulley for the whole machine 
@Martin_Bleakley are you using 1/32 microstepping?
*calculations above may be incorrect - I only think I know how steps / rev works
If the motors are .9 degree/step then yes 400/rev if they are standard 1.8 then only 200. Then the 2:1 because of the pulley setup. With .9 degree steppers a 1:1 could be maintained or minimum layer height halved by continuing to use 16t w/32t
tim im using 1/16 and im using the rumba electronics http://www.reprapdiscount.com/electronics/54-rumba-board-incl-6-a4988-driver.html the steppers are http://www.aliexpress.com/item/5pcs-NEMA17-78-Oz-in-CNC-stepper-motor-stepping-motor-1-8A/523595194.html
what about 36 teeth on the motor (5mm bore) and leadscrew (8mm bore)? this would be a 1:1 ratio. This is off stock, just that I have to drill the bore to 10mm.
If that’s what your wanting to do then go with it, ratios are more a persons personal preference than anything