You basically need to be saving 33% of your take home salary.

As a rule of thumb you shouldn't be spending more than a third of take-home on your housing. That leaves you with 33% for your cost of living.

To see how that breaks down in "real-money", the take-home of an average CS graduate working at a typical large company in a developer role in London would be around 20k.

So that would be around £550/month (£6600/year) in rent, so a shared flat in a cheap-to-average part of town.

Food would cost you about £2000/year

Personal goods another £2000/year

Travel around £1200/year.

Leisure around £1400/year.

Leaving you with £6.6k in savings, enough to afford all of the above for another six months without pay.