I would recommend ringing the hmrc helpline to ask for guidance in this regard, hmrc have been known to have made a few incorrect in year coding adjustments 17/18 and 18/19 that could and probably should be removed - i have know idea if this is the case for you - but it is a possibility.
Normally i would expect to see specific mention in 17/18 code that there was a potential underpayment being coded the following year - this may be on the notes on the december 17 code change even though the adjustment there is reducing your tax code.
Note in the absence of you confirming with hmrc they will likely amend the return to match whatever is on their computer whether that is right or wrong - so if you choose to pay 276.02 hmrc may simply remove this amount from the calculation 5/4/18 on the basis they expect you to pay it ye 5/4/19 - if thats the case ensure you add the amount of tax due to you 18/19 calc.
Hopefully you will get suitably trained person who can explain everything in a manner that makes sense.
Hopefully hmrc haven't further complicated tings by doing p800/simple assessment calculation in error too.
Note if you do need to include "tax coded out to following year" on the 17/18 year - it should be the amount of tax at stake that you enter on the return NOT the coding adjustment - this being the actual tax mentioned as being transferred between tax years and not 862 amount of code change - i suspect you have included the 862 in your adjustment when comparing the two calcs.
Ridiculous how hmrc can make an easy self assessment calculation become stupidly complicated simply by messing with the tax codes when there was no need.
Final note for future reference there is a box you van tick to say "don't code out future potential income sources in year" - normally the hmrc adjustments apply when they know individuals have dividend income/rental income and they don't tick that box - if you tick the box the "presumed" tax due based on prior years will be left off the tax code leaving things to be sorted on the tax return.