The pot size which determines what you’ll have in retirement, is the pot size when you retire.
So £300k when you retire might get you £10k over year. You won’t have put £300k in as some of it will be investment growth and tax relief.
So £100k now would get you just over £3k if you don’t put more in and it doesn’t grow. Those are figures worth knowing, so you can map ahead to see what you might need in retirement.
Some of the websites linked to suggest how much money per year singles or couples need for basic/comfortable/luxurious retirement. That stuff is useful too. It can give you a target.
If you have a target and you know what state pension you’re on track for, you can see how much private pension income you need per year to achieve your target. Then you work backwards to see how much you need to put in over year to achieve the pension pot size needed.
Things don’t always work out exactly as planned, and sometimes you have to accept you won’t pay in as much as really needed to achieve your target, but having the knowledge and acting in it is very empowering and liberating.
OP, you should have a look at the sites and consider what income you might want in retirement.
Look at your state pension forecast.
Consider how many £k your private pension needs to give you per year.
Think about the pension pot size needed if you need £30k for each £1k pension per year.