What are you asking exactly? If it's OK to use that spare for emergency use? If so, that will not be a problem. The viscous will cope untill you get the flat tyre replaced.
If you want to run 1 18" and 3 16" tyres all the time, then I'd say that's not a good idea. There's the small difference in diameter, which will in reality be even greater because the spare is new while the others have already lost some thread, so a constant load on the diffs and viscous.
The tyres will also behave differently when driven (different sidewall, possibly different compound, thread, ...) and while that might nog be noticeable under normal driving, it could make a difference in an emergency.
Both standard sizes so very little difference in rolling circumference so no different to having a mix of new and worn tyres. If you use the spare as intended, purely to get you home, then not a problem but as Filip says, not a good idea to mix different tyres for normal use anyway.
Sorry for being the curmudgeon here but why are we going down this path?
The correct answer is NO. Never run different diameter rims and or tires (tyres) on your Rovers.
It will damage the transfer box. While I understand that it can cope with this for a few miles it's never a good idea and should always be avoided.
All things being most ricky tick we really should be rotating out our spare with our other tires anyway right?
Strictly speaking you are right but, and there is a but, he is using a standard size wheel with a standard size tyre. Some cars came with 16" (in both 7 and 8" width) and some with 18" but the tyre size was chosen to give the same rolling circumference so the speedo still reads correctly no matter which of the 3 wheel/tyre sizes was being used. The difference in rolling circumference between the two options he has is only 0.03 of an inch in 91" (or 0.033%). You'll get a far greater difference than that between a new tyre and a half worn tyre so the diffs and viscous coupling are designed to cope with a minor difference. Land Rover don't advocate rotating the wheels and some European cars spec cars came with alloy wheels but a 16" steel spare (even when the car was supplied with 18" alloys) it wouldn't be practical anyway. It's no different, in fact preferable, to using a space saver spare that most modern cars come with anyway.
I thought my term "diameter" was sufficient to convey my thoughts. I'm sorry if I was misunderstood. I can't read the chart that is in the OP's post.
A few hundredths won't make a real difference no.
Personally when I buy tires I buy 5 and I keep my spare in the front to back rotation.
I've never had a need to change my factory rims but if I did and they were a different size for permenit road use then I would buy 5.