WHOA! - what you say, Sir R, is correct if you are using a sea level datum pressure such as area QNH or 1013.2/29.92. In fact, since Station QNH is ALREADY corrected for ISA deviation (by the clever met observer and his/her abacus), the vertical airspace we need to consider for OUR error is HEIGHT above datum, ie 10,000-2,500=7,500ft, to which the 4%/10 on the -15 error should be applied. Thus the answer is 9,550' (You
are, of course, safe!). Also, for guest33, be careful as the question asked for your actual altitude (not 'height'), so do not take the station elevation OFF the 9,550. NB also the other 'catch' for the unwary in these dreaded questions is when they try to 'trick' you by telling you the Station QNH is 29.92in. Same answer.
Of course, in practical cockpit terms most of us do it your way for simplicity - and to err on the safe side (Dead sea and AMS excluded