I would start off thinking about your ideal discretionary spending and working backwards.
I definitely wouldn’t go for a % approach - don’t fall into the trap of thinking that you need to spend more if you earn more.
You’re perfectly happy on current salary, right? So you could actually save ALL salary increases. You don’t need to spend more.
So, ideal discretionary for me:
- theatre once a month, but happy with cheapest seats
- cinema once a month
- hair once a month
- holiday once a year at £2K budget / 12
- 2 weekends away, budget £500 each: £1K / 12
- gym membership (cheapest)
- phone (2nd or 3rd latest so £££)
- able to eat out 2x per month
- clothes: not a big deal for me, so more a case of annual /12, I wouldn’t buy clothes every month
- £50 of “just because” money so I get to feel extravagant without blowing my budget. This month, that would have covered me deciding I fancied a new badminton racquet
Be honest with yourself about what you would LIKE to spend.
Nowhere on my list are things like make up and take away coffee.
Then you can start looking at % and what is left - and if it’s not enough, start prioritising.
I would recommend keeping your treat money in a different account. That way, if you under spend then you start to accumulate treat money - so under spending becomes a reward.
One thing I would say - I earn a good salary and I love being able to get things (I have a child now - so mainly for her!). But... I was just as happy on half my current salary. You do start to spend up because you can. I was just as happy with my cheap travelodge really, as with the boutique hotel. Looking back, I kind of wish I had saved EVERYTHING over my salary of 10 years ago. And I am a saver so it’s not that I regret frittering it all away
Just - I could have retired earlier. But that depends on your goal. It’s only at 50 that I’m really thinking it would be nice to retire early. At 30 I thought “I love my job, why would I stop?!” 
All the above aside... as soon as you hit 40% tax bracket, work the hell out of pension tax relief whilst it’s still in place!