Unless you have a problem, don't prematurely optimize would be my advice. The hardware will run at 100% usage (or 800% of one single thread in your hardware's case) with no ill effect and unless you have blocking effects - your animations and kiosk functions might not even degrade when the CPU is totally busy 24/7.
If there are low hanging fruit to reduce CPU, go for it, but the i7 has 4 physical cores and calculates CPU loading against 8 virtual cores. Your hardware goes to 800% ideally and you are only at 150% presently? Seems like you specifications for hardware are more than overkill for a single use kiosk situation. If you have chosen a technology stack and are happy with the contents of the kiosk in terms of animation, ability to perform the task you are "hiring" it to do - I wouldn't worry about some externally imposed "best practice" even if it's Apple's guidelines to be an efficient coder (which are EXCELLENT by the way).
On any system, you might want to plan to not exceed 80% utilization if you cannot benchmark, measure, react and correct burst usages. This is a guideline, and of course when you can do better, you should.
I'll assume you actually have remote access tools in place to push updates and can measure the CPU over time and react, but kiosk interactivity seems low on the "we lose money" when the system lags.
- How long does a user interact with the kiosk?
- Is money being made or lost and at what rate is that estimated?
- What happens to the system when it becomes CPU bound (due to background processes or change in the app)?
Any "best practice" really is a starting point for you to evaluate things. There's no reason to not leave a computer running 100% utilization if the main thread isn't blocked and the system as a whole degrades gracefully.