The Local Housing Allowance for a person living in shared accommodation might be a good guideline. (That's the amount which the local authority deems is sufficient to pay their rent). Here in Mid-Suffolk, that's £57.34 per week, which is equivalent to £248.47 per month, so Bednob's figure of £250 per month (on a 'rent only' basis) looks reasonable to me.
To find the LHA figure for your own area, see here:
https://lha-direct.voa.gov.uk/search.aspx
However if you're adding on food, laundry, etc, I'd probably suggest a figure somewhat higher than that offered by Bednobs, at around £400 to £450 per month.