Let the fun begin !!! ???

[EddyCurrent]

this is my z axis end view, it has 20 mm rails and a travel of 160 mm. i'm thinking of a z axis ballscrew length of 400+ 30 for for a timing pulley. i'm sure this is enough or should i go more?

If the backplate is 400mm same as the spindle mounting plate you will likely want the outside faces of the bearing blocks to be 400mm apart so they are flush top and bottom, this will allow the maximum travel.
So the BK bearing is 25mm thick (excluding the end cap and locknut) the BF is 20mm thick, so that gives a ball screw length of 355mm between shoulders. Then 12mm turned section on the BF end, 69mm on the BK end (E+F = 39+30). Total length = 436mm. You have to allow for the Z ball nut thickness , this will reduce the overall travel, I drew mine out using Sketchup to make sure I was getting the expected travel.
Also you might as well get the ball screw as long as possible because the price difference for a few mm is going to negligible.
I know it might slow the build but in my case I waited until the ball screws arrived before cutting metal. It was easy to size the metal to the ball screws but impossible the other way round if in error. That's one of the reasons I built the electrical panel first, waiting for the stuff from China.