Schengen rules

Your calculation starts as soon as you enter ANY country in the Schengen area which includes both Italy and Spain so all trips will count on a rolling basis across years rather than a fixed 180 day period in any given year.  The calculator link below should help, just enter your dates for starting and finishing your trips and it will tell you the number of days left...

https://ec.europa.eu/assets/home/visa-calculator/calculator.htm?lang=en

Get a calendar, mark on when you travel and the days out of UK.  Count backwards. So you can spend 90 days out of every 180 days. 

You leave Spain on 11th October. Count back 180 days from that date. In that 180 days you are only allowed to stay for 90. 

So basically I am in Spain from 4 April to 1July 2023. What your saying is that I count back from 1 July for 180 days and within that period I can stay from 90 days. Therefore the Spanish trip between  6 Oct and 11 Oct is irrelevant  as it is outside the 180 day period from 1 July. Hope this is correct.   
thanks 

Not correct!  Your earlier trip is relevant. If you count back 180 days from 11 October that date is 14 April. Therefore you need to count the days you spent in Spain since then which on my reckoning is 85 (79 + 6) so you are still within your 90 day allowance. Just!  At least that’s my calculation and understanding of this rule. 

You count back 180 days from 11th October and you count back 180days from 14th July. 23 the 180 days is always rolling. Once you have counted back the 180 count the number of days you have been in the zone. If you are over your 90 you would have to re arrange your travel plans. Keep counting back from every return home date. 

But what will happen if you overstay by say 10 days. Will border police have time or inclination to do the maths

James. 

Yes, they will. And your really don’t want an “Overstay” stamp in your passport … makes getting back in a bit tricky as they can refuse entry.

Quite apart from anything else, aren’t passports scanned at entry / exit? If so calculation is done automatically 

Peterlee4 has given the best response here - don’t rely on counting back manually on a calendar - use the free Schengen calculator and save yourself a great deal of time and manual calculation, and get a solid indication of how much time you have left in the Schengen.