How is ‘Adequate Maintenance’ calculated?

If someone is recieving disability allowance then they only need to demonstrate adequate maintenance to sponsor their partner but how they calculate this for partner applying for fiance visa and 3 dependent children who are british national living with sponsor

Sponsor is getting state pension, disability allowance, housing benefits, child tax credit, child benefit etc Do these all benefits count toward net income if there is no others source of earning

I understand that formula to calculate the adequate maintenance threshold is total income minus rent/council tax which should be more than the income support level for a family of his size but what is income support level of one couple with three British dependent children and what if council pay rent and there is no council tax. Do he will need to show 6 or 12 months bank statements along with benefits letters?