Free Seating
 

How do you work out the stab trim required on a 737 such as Easyjet/Ryanair when there is free seating; simple answer I'm sure, but I'm puzzled.

Good question and probably not one to answer easily!
Generally when 'free seating' applies then rather than trimming the passenger distribution by either 'seat-row' or 'cabin area' a computer system will trim by 'class' using the class centroid to calculate moments (index effects). Additionally under such conditions a more limited balance envelope (i.e. more restrictive forward and aft limits) will be applied. Then the usual MAC calculation will take place based on the laden index at TOW to compute the STABTO.
The same principle will appy for manual documentation and the use of an index table (rather than drop line) to calulate passenger trim.
Of course both systems are based on the assumption of 'even' passenger distribution about the class (cabin) centroid; if distribution is not even then there will be a margin of error; however due to the reduced balance envelope, that error should not usually result in more then a (+/-) 1/4 unit (0.25) on the STAB which is within tolerances laid down by the manufacturers; e.g. Boeing allow for a 767-300 +/- 5% MAC when determining the HST setting, which is quite considerable.

Gosh, understood about one word of that!! I think that the key word that you used was 'assumption', but maybe it doesn't make thay much difference....or does it?
Thanks for taking the time.