This is the calculation:
Standardised score = (((raw score - mean) /standard deviation) x 15) + 100
Total score = 1.5 x standardised Mathematics + 1.5 x standardised English. (from a thread from 2014)
And here are the mean scores and standard deviations for the 2018 entry papers (from the 2018 collated cut-offs thread):
English (% since from 2012 to 2014, the paper was marked out of 50, but from 2015 out of 60)
____________________ 2012 ____ 2013 ___ 2014 ____ 2015 ____2016 ____ 2017
____2018mean ________________46.0____ 37.4____ 49.3_____ 55.3 ____ 50.8 ___ 47.2 ____
67.1standard deviation ____ 13.8____ 14.2____ 13.8_____ 14.4 ____ 9.2 _____8.1_____
8.767.1% of 60 = raw score of
40.26Mathematics (% since from 2012 to 2014, the paper was marked out of 40, but from 2015 out of 60)
____________________ 2012 ____ 2013 ___ 2014 ____ 2015 ____2016 ____2017_____
2018mean ________________ 54.1____ 54.6____ 52.1_____ 63.9 ____ 59.9 ____58.3_____
54.1standard deviation _____ 20.4____ 17.7 ____ 20.5 ___ 17.8 _____12.8 ____ 12.0 ____
11.254.1% of 60 = a raw score of
32.46Applying a bit of trial and improvement, I calculated that
• an English score of 36 plus Maths of 36 would have given a standardised score of 296.094
• an English score of 36 plus Maths of 40 would have given a standardised score of 304.13
• an English score of 38 plus aMaths of 37 would have given a standardised score of 303.275
(I think! - feel free to chip in and tell me I must have gone horribly wrong somewhere

).