Few things:
As a general rule, don't rely on Johnson Rods' recommendations. The limiting factor for almost all tire sizes is turning radius, frame horns, ac/heater lines, and what happens if the LR4 drops to the bump stops--things the rods don't change.
The strut spacers alone will not change the height of your vehicle. The height of your vehicle is solely determined by the orientation of the suspension sensors (which can be tricked with the rods) and the suspension ECU. If you install the strut spacers without changing either one of those things, the suspension will adjust itself to ride at the stock ride height.
The main thing accomplished by the SYA kit is making it so the vehicle sits higher when it's dropped to its bumps, which ameliorates one of the issues in the first paragraph I typed above. You'll still have to contend with the other issues.
As someone who has done this, I would recommend understanding what you are getting into and having the necessary tools to get to a workable setup.
IMO, do not do this unless you have both the rods and a IIDTool to fine tune your setup.
@ryanjl is correct in that if you install the strut spacers with no other modifications, your truck will be sitting at exactly the same ride height that is was prior to installation. The question you should be asking is how does this happen with an additional 2 inches of steel in the equation, and the answer is that the truck inflates the airbags at each corner to a lower pressure at each ride height that is selected. The real world effect of this is that the truck will ride like an unsprung horse cart at standard height.
Inserting the rods into the equation provides the suspension controller with a reading that is somewhat akin to what it expects to see. Thus, at standard ride height setting, you are approximating an airbag pressure somewhere around standard height in an unmodified truck but are now sitting 2 inches higher thanks to the spacers.
From this point, I would recommend using the IIDTool to fine tune the ride height to your preference based on ride quality you can accept versus suspension travel you are not willing to give up. If you search some of the older SYA spacer threads here, there are some discussions that go into more detail. I don't have my notes handy, but I know I posted what I settled on as my finished suspension tune in one of them.
I really like the suspension on these trucks, but there is no free lunch. If you are going to get something, you are going to give something else up. Just know what that is, make sure you are willing to make the compromise, and understand what you might have to deal with in the future from doing it.