How many NATO countries actually step up.

Yes. What I get is you want to limit spending on defence by claiming an upper limit which creates this barrier. 
But what if I make the prudent spending limit 16? Can I then spend the extra 4 on defence without limiting spending on health or do I now need to spend another 4 on health? 
Sadly the real point I was making was lost in your desire to stop spending on defence. 
That point was as much about fixing the country as building our defence. 
Lets try leaving defence out of the conversation altogether for a couple of posts. 
Do you agree the country is in a hell of a mess. 
List: 
1, NHS in bits understaffed and derelict.
2, Climate change costing more to combat every year, whether that be in prevention or dealing with its effects.
3, Schools crumbling and education failing our young. Teachers leaving in droves and being replaced with barely trained people on watered down courses.
4, The roads and infrastructure worse than anyone can remember and even the new wind farms not connected to the national grid. 
5, A huge housing shortage driving millions of pounds into the arms of the rich and landed. 
6, Chronic depression and anxiety among the young driven by a lack of hope in any future. 
7, The economy stalled after Brexit, hit by Covid and crushed by interest rates (Why are the young with mortgages and rents responsible for fixing inflation) 

Anyway I feel we are in a national emergency not dissimilar to a war. And that to stop it dragging on for decades we should treat it as a similar emergency as ww2 and borrow  more to get it fixed. In the hope that a stimulated economy will give hope to the young and make enough money to pay back the debt. 
In ww2 we borrowed 275% of GDP, I'm not suggesting anywhere near that amount. 

How does that sound? 

So to you a higher limit to your arbitrary one means unlimited? 

Perhaps you should look up the meaning of the word. That or stop trying so hard to be a clever dick. 

Unnecessary military spending is only unnecessary until the moment you need it, by which time you discover you lack the weaponry and armed forces you need. We went down that road in the 1930s. If there is a great power throwing it's weight around - and there are potentially two if you include China, which is probably the more dangerous of the two - in a situation where military support from the USA is far from certain due to it's own internal politics, it clearly would be most rash not to recognise the potential dangers.

And history tells us that we are far less likely to have to fight wars when we demonstrably have the power to easily win them. At the moment USA support cannot be relied on with any certainty, whilst Europe itself is less than entirely united. There is of course the inherent danger that Putin is waiting for the US electorate to impose a massive defeat on the west by electing Trump, who is more likely based upon his past rhetoric to want to be more friendly with Putin than he is with NATO. At which point the US power behind NATO ceases, and the alliance becomes much weaker.  If Trump wins the west could be in big trouble. We need to try and be ready for that. Because the increase in military spending that would be necessary if we found ourselves at war would dwarf the spending increases necessary to deter it.   

Well said, very good points.