You appear to have plucked this £400 out of thin air!
If you have £1500 to spend every month, are you saying that you would spend £1100 and save £400, so that you have savings to cover inflation when prices go up?
That would work for a couple of years - it’s not really inflation-proofing your income, it’s just making sure that you have savings so you can cope. But long term, that’s not going to work. If you saved £4800 (12 months of £400) then left it just sit with little interest in a bank account, then by the time you needed it, your £4800 would be eroded by inflation and won’t buy as much as it would have done in the year when you saved it.
It would help if you could be clearer about this £1500 income, where that’s coming from.
The ideal situation is that you grow the amount of money that you have (by investing it) so that it increases by more than inflation. Easier said than done! But the minimum you want to do is have enough growth that your money increases at the same level of inflation.
If your £1500 is coming from you having cash in the bank and planning to withdraw £1500 a month, you might want to look into buying an index linked annuity product. This means that you buy a product that pays you £1500 a month. But the “index linked” part means that every year the £1500 is increased by a specified measure of inflation. So if that measure is 2%, then next year you’ll get £1530 a month. So your income goes up with inflation.
These are complicated products and I’m not saying that you should do this. What I’m saying is that it sounds like you’re just taking a guess at what you could set aside, when you might be better to talk to a financial advisor.
I looked at your other posts (it can help to answer a question if there’s more info) and it looks like you’ve just bought a £185K flat without a mortgage. So it sounds like you have quite a bit of money at the moment, and would really benefit from proper financial advice.