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No, 532mm would be fine. If your blocks are 16mm then 532mm is the measurement to go for.
I mounted the side plates to the linear blocks, square everything up and 631mm is what I measured. Maybe I should have been 632mm as a very slight deflection over that distance would have made the 1mm difference.

 

I’ve been using mine for almost 12 months now. I went with a Makita RTO700 as a spindle as I had one already.
I’ve added a few more things since the build. I built an electronic switch so I can use gcode to switch the router ON/OFF. I have added a 5.5W laser and will probably upgrade that later. I use gSender so I can software switch between the router and the laser.
I can now control the CNC via a Bluetooth Xbox controller which is great.
I decided to try and machine aluminium and it does it fine, using the right bits.
I looked at adding a spindle and VFD but the spindles weigh 2.2 kg. The router works fine but you have to adjust the speed manually which really isn’t a problem

 

I thought about that but I’d want 8 or 10mm side plates rather than the 6.4mm. I got a quote to water jet cut a set of 8mm plates before I bought the plates from Chris and it was about the same price but I needed the dxf files to get exact hole location etc. so I didn’t go ahead.
One thing I forgot to mention is I added tension nuts and bearings to the lead screws on the X and Y lead screws. I was having problems with the inner stop collars moving which put a little freeplay in those axes. Certainly made a difference but I had to buy all new lead screws as I needed extra length for the tension nuts so that’s something to think about before ordering cutting lead screws to length.
I will probably have to modify the Z axis to get access to the anti back lash nut adjustment. You can’t get to them when assembled.

 

Edited to give correct dimensions

Re tension nuts and thrust bearings - you need a set at each end of the lead screw. The go on the outside of the plates.
The ones I bought consists of a brass nut ( with locking grub screw) and thrust bearing set(consists of a thrust bearing and two thrust plates).
Re Z linear rails - I used 250mm. My plates are 11”. The X gantry sits 7.5” above the 8020 extrusion base and I use a 0.5“ base board attached to the extrusion. I then have a 17”x17”x 0.5” spoil board fixed to the base board, so in effect it’s 6.5” from spoil board to bottom of the X gantry. The 250mm linear rails gives me JUST enough down movement to use a 25mm diameter “plate” bit to flatten the spoil board because the Makita sits high (I use two mounts for rigidity). If I used one mount, I’d have plenty of height/movement all the way to the base board.. If you’re worried, go 300mm but you are not really going to gain much use of the extra length unless you are using very thin spoil boards..

 

Here's some links to the tension nuts and bearings I used. I'm in Australia so the shop may not be relevant to you

Thrust Bearing set
Thrust Bearings - Maker Store PTY LTD

tension nuts
Lead Screw Tensioning Nut - Maker Store PTY LTD

Please note that I edited my reply above to give more dimensions and info.

 

I’m on a tour at the moment.. I should be able to reply with a detailed answer tomorrow.

 

I aligned a top linear rail on one side to a set distance between the top of the side C beam and the side of the linear rail and tighten the screws fully. I then attached the two top blocks to the side , tightened the block screws making sure the plates slid freely from the back to the front.
I then attached the bottom linear rail and left the screws loose so I could move the bottom rail up and down a tiny amount.
I the attached the two Botton blocks leaving the screws loose. I then moved the plate to the back and tighten the back linear rail screws, but not very tight. I then slide the plate to the front and tightened the two front linear rail screws, but not very tight.
I then moved the rail to the centre and tightened the centre linear screws but not very tight.
I then checked that the plate slid easily from to back,
I then tightened all the screws to final tension and once again checked that the plate slides easily front to back.
Now comes the time consuming part.
Tighten the lower block screws, one one block, one at a time, checking after each tightening that the plate slides freely. If you tighten a screw and the rail gets hard to move, you will need to shin between the plate and the block near that screw. Continue on the block so when all the screws are tightened the plate slides freely. I found some need a shim around the top screws and others between the bottom screws. Then do the same for the other block.
Once you have one completed side sliding freely, do the same to the other side.
I had to do the same on the X axis.
It took me a few hours from memory.

 

This is the shim packet I bought
Steel Shim Assortment 150Mm X 150Mm (4 Sizes) | Assortments - Shim Stock

I mainly used the two smaller sizes
I would use auto Y axis squaring if it is available I normally check mine before I start a job by homing and measuring with a vernier.
I’d be interested in the controller your using.
Mine is a self built unit based on an Arduino Uno board