Easiest way for me to think about it:
If you wind your Altimeter UP to 1013 from the QNH (eg 990) then you wind your altitude reading UP by "27ft per mb" and are therefore HIGHER on the altimeter - then you just round (ALWAYS UP) to the nearest FL
And vice versa. If you wind it DOWN from (eg) 1023 to 1013 then your altimeter will read a lower reading which you round (ALWAYS UP) to the nearest FL
So first work out your altimeter reading after setting 1013 and then round up.
(This is very much clearer with the altimeter in front of you - I think I've hurt my brain again!)