The calculated distance will vary depending on what speed schedule the operator chooses to use. A faster speed will give a greater distance, but will mean a lower maximum altitude & therefore may give terrain clearance problems, depending on the area of operation for that particular operator. The distance obtained from these calculations by the operator will then need to be approved by the regulator.
Once the operator has the distance approved this becomes the EDTO threshold distance (ETOPS is now referred to as this - Extended Diversion Time Operations). This figure does not change from day to day, as it is a theoretical figure. Therefore the actual time to fly from this point on one engine on any particular day may be greater or less than 1 hour due to wind & the actual speed used on that particular day. The reason I say 'the actual speed used on that particular day' is because you are not obliged to use the nominated speed that was used for the regulatory approval process. On the day you can fly what ever speed you feel appropriate. Just don't run out of fuel!
So this statement -
If a twin engined aircraft is flying more than 60 minutes away from a suitable operational airfield, with this time being calculated for a single engine operation in still air conditions, then the aircraft is flying ETOPS.
is incorrect. The aircraft is EDTO once it passes the EDTO threshold distance in the operators manual. Most use the fix page to put the EDTO threshold distance in & therefore have the threshold distance displayed on the ND as a circle around the EDTO adequate airport/s.
It can be a little confusing as the EDTO threshold is referred to as a time for convenience, when in fact it is a distance. Time & speed are simply used to calculate it. To further complicate matters, the various EDTO times (such as 60, 120, 180, 207, etc) can be calculated using different speeds by the operator, which means that the distances for these various EDTO times are not multiples of each other. For example, my company uses 431nm for their EDTO threshold distance (60 minutes) & 823nm for their 120 minute distance.