Z axis design

Hello everybody,

I try to build my first CNC with working area 900 mm x 600 mm x 150 mm.
At the moment I started with Z axis design but have a small problem. I plan to use Hiwin linear rails and have found that this is impossible to use 15 mm rails because corresponding bearing blocks will be very difficult to grease. But standard bearing blocks for 20 mm rail (HGH20CA) large enough (length with grease nipple is 90 mm) and if use 2 of them per rail then Z axis should be really long that to have sufficient travel length. If to count that 30 mm from each side is reserved for bellows then I will have only 100 mm travel for 360 mm rail.

http://www.mycncuk.com/attachment.ph...id=14095&stc=1

So necessary to add at least 60 mm to rail length but then I think this Z axis becomes too large for CNC with such working area.
Then I replaced 2 standard blocks with 1 heavy load block (HGH20HA). These blocks have greater length and rated moments.

[HTML][HTML][PHP][HTML][CODE]http://www.mycncuk.com/attachment.ph...id=14094&stc=1

And now I have 180 mm travel with the same rail lenght.
But as I newbie I'm not sure if possible to use only 1 block per rail even if this long block. Can I use only 1 such block per rail for Z axis?

This is part from techical documentation where you can see parameters of blocks

http://www.mycncuk.com/attachment.ph...id=14096&stc=1