Income Tax & marriage allowance

Dear teacher, 1. Is there an extension to the proforma tax computation to calculate the tax% on each income type? I found myself mixed them all up often. Use Chap 2 P9 Example9 as an example, my calculation is: 130000+40000+36000= 206000 additional rate ------------------------------- (37500) -- 20% = 7500 ------------------------------- (150000) -- 40% = 60000 ------------------------------- (1850 ) -- 38.1% = 704.85 After checking the answer, I realized it was wrong when I allocate the % to the correct figure. but I struggled with it. Will you please guide me? 2. I don't fully understand the marriage allowance. what does it mean by the £250 cannot create tax repayment? if it's already reduced the tax liability, why do we want it to be returned to us? are there situations that we may want the tax returned or to be credited to some amount that will reduce the payment to HMRC? and also the 4 years time frame. will you please give me some examples to understand better? can this deduction be backdated within this 4 year time frame? Thanks Lyvia