Second Monitor Calibration

Just added a second monitor (Acer K272HUL) to my Imac

27" mid 2010 with 10:10 just loaded.

ATI Radeon HD 5750 1024 mb

To a point all is working well addaptors cables and configeration get a 'tick'

However calibration and resolution are the issue.

First if set as an independent monitor - the 'best for display' resolution is 1920x1080 (not the 2560x1440 the new screen is capable of)

if set as scaled the best available on the drop down list is also 1920x1080.

I think this is solved by 'grouping' the monitors as when I do the new screen is of comparable resolution to the main screen.

At this point all would be well however when I calibrate the new monitor under gathered it obviously now changes the calibration of the Imac too.

So any ideas how to get the new monitor to work at 2560 x 1440 and be indipendently calibrated?