Paye discretionary bonus tax double check

I am on paye and started a new job March last year with a large pay rise to 84k gross salary. Looking at my hmrc account I can see that my taxable income for this tax year should be 75k based on my salary monthly bits on the account and my hmrc estimate is 80k. Both these seem right as it accounts for pension and other deductions on my salary and the other way accounting for untaxed savings estimate.

We have been told there is discretionary bonus this year payable in March this year with the option of salary sacrifice into pension. You get to set the maximum amount you take as bonus with the excess going into pension. This has to be done before we know the amounts.

I have read about the 100k tax trap and the 125k additional threshold. I have done some calcs adding up taxable salary+ savings + medical benefits and can work how much I could take as bonus to be under 100k with a buffer.

I am assuming that the bonus amount would be taxed wholly at 40%? What I am trying to understand is if everything is fine I shouldnt get a tax bill come the end of the year for underpayment in tax due to this bonus?