Tax payable in retirement

Is there any formula method to enable you to work out how much tax you pay on your state pension and works pension are combined as I need to know the take home figure will be after tax deductions, am I correct in thinking that your state pension and works pension combined if below £50,270 will only pay 20 percent tax. The formula I have been using is ie - The works yearly pension added to the state pension = equals , then take away the £12,570 personal tax allowance from this amount = this equals the amount you will have to pay tax on, then times this figure by 20 per cent , this will then give you the total tax to pay ,then minus this tax to pay figure from the first 2 figures added together , then add back the £12,570 to the last added figures together . This will give you the annual tax free figure for the year. I need to know how much to budget for when I retire and this was the only way I could think of working things out. Please correct me if I am wrong and if you could correct the formula I would be most grateful.

There is not a forumula as such, however you may find the following guidance links of use: 
Tax when you get a pension: What’s taxed - GOV.UK
National Insurance and tax after State Pension age: Overview - GOV.UK
Thanks