Is the money you're paying towards the lease, or is it the tax you're talking about?
Your BIK should go through the calculation. You can also use online tax calculators to check what tax you should be paying on your salary normally, and the added amount for the car on top.
The problem with company cars is that there's often a delay between you getting the car and starting to pay tax on it, so you're often behind, which leads to a period paying more tax than you otherwise would, possibly at a higher rate, while you catch up. And as you have found, it can be hard to know where you are. But if you log into your HMRC account and look back through your last few tax years, you might be able to work out where you are.
But anyway, the tax due is:
BIK value x % for CO2
eg 77000 x 0.12 = 9240
Then you can deduct any money you pay towards the having the car, you say you have £1200 taken out of your salary each month - is this for the lease? Surely not? But then if it's a £77k car, it might be right as 1200 x 12 x 3 for a 3 year lease is £43200, which could be right, especially if you're quite a high mileage driver and it includes maintenance, and possibly a contribution towards insurance.
Which leaves the amount that you pay tax on, but this would mean no tax is due as you're already paying more than the BIK.
But if the £1200 is not towards the car, I don't know.
But anyway, if you do still have a taxable benefit, this amount should be divided by 10 and deducted from 1257 to work out the effect on your tax code.
To work out the tax due, multiply the taxable benefit (before you divide by 10) by your marginal tax rate.
But going by the value of the car, and your comment about not having a personal allowance, it sounds like you're quite a high earner. In which case, it's probably worth asking an accountant for advice and to complete your tax return. It shouldn't cost very much (£1/200?) and at least you'll know you've paid the right amount of tax.