Custom CV Axle Length Sizing

I bought a project B13 Sentra rally car that I'm getting ready for stage.  In getting it prepped, I found some suspension issues.  In fixing the suspension issues, I found some more suspension issues.  So here we are:

At full droop with the longer struts that are on the car, the inner CV is close to falling out.

I over thought a lot of ways to fix it (new strut turrets, limit straps), but got some advice that maybe it's as simple as getting a longer axle.  So I took the spring off one corner, jacked the suspension until the axle was parallel with the ground (the point where the axle is "longest"), then took the nut off the hub side and slid the axle in as far as I could.  I measured about 40mm of additional inward play in the inner cup.

This is with the axle parallel to the ground:

This is parallel and pushed all the way in against the bottom of the cup - about 40mm:

My question is - what is the typical way of sizing a CV axle?  All other things being stock (control arm, knuckle, etc) - why did Nissan leave 40mm of inward travel available, and how much travel is too little?  If I just get an axle that's 20mm longer and it doesn't bind anywhere, is that good enough?  Or am I making this too simple?

Sway-A-Way has instructions for measuring axles on their website.  Basically, you measure with the suspension at full droop. You want the axle long enough to put the center part of the CVs right in the middle of the outer housings.  Then move the axle up so it is horizontal and make sure that it is not so long that it pushes the center part of the CVs all the way out to the CV flanges and binds.  That's it.  

Much good CV info here

It is weird that you are extended like that at full droop.  Usually the axles are longer than the control arms, with the end result that max extension is roughly around when the arms and axles are straight out and max plunge is at full droop.

I think that Altimas have the same style control arms but longer, I wonder if Altima axles are the same splines but longer.

I had custom half shafts made for Midlana. The manufacturer, The Driveshaft Shop, only needed to know the total length "flange to flange." Probably the best way is to ask the manufacturer directly.

You need to find out what 'plunge depth' your suspension exhibits.

and what 'plunge depth' the current (and replacement candidate) axles provide.

At some point, a DPO may have replaced with an incorrect axle that 'fit', but does not always 'work'.

In general, as said above, you are aiming for those distances to be centered at normal expected ride height.

The axle plunge handling mechanism strength limit (ie load handling capability) will likely be lowest at full extension.

For example the ends of the 'tulip' on your axle will see the greatest bending loads in it's walls and structure,
whereas the fixed tripod and roller bearings will see the same loads no matter what.

OTOH, with axles that employ sliding splines, the axle tip will see greater torsional deflection.

In your intended rally application, you can expect frequent very high transient shock loading at high angular deflection
that will shorten service life and increase the likelihood of failure.

Awesome, those are the answers I was looking for.

I ended up finding that a 99-01 Sentra with a 2.0 has the same splines but is 20mm longer.  I think it's going to suit my needs on the driver's side.  The stock passenger axle seems okay.