It will include the previous spend, yes – so it should show £160 remaining.
Possible glitch?
If my contactless until PIN entry is set at £200 (say 150 left) and I then lower the limit to £100, you would expect it to now say 100 left. As the limit is now below the amount left before (when the limit was higher) However the left now says 50.
Apologies if I’ve missed this, but what happens if you disable contactless or try to exceed the limit? Is it a straight decline or will it say insert card + enter PIN?
Pretty sure it’s explained further up, it’s not a glitch that’s how it’s been designed.
You’ve used £50 of your “pin free” allowance both before and after you edited your upper limit.
The amount left will be your new limit minus what you’ve already used. That’s how I would expect it to work.
You’ll be asked to pay with Chip & PIN, yep!
It’s the same as exceeding the limit today, it’s just that we’re letting you decide the amount that triggers needing to enter your PIN. Hope that helps 
We’ve got an update regarding this feature over here. 