External monitor

Your guidance please. As I'm mainly a stills photographer I've recently simplified my system to an iMac with a Thunderbolt Display as a second monitor. This means I no longer have the Dell monitor I used to use with my Blackmagic Extreme - and Premier Pro. If I just use the Thunderbolt display as an external monitor using Premiere's built in playback feature, I don't get a scaled, pixel for pixel output.

Is there any way, please, to use my BM Extreme with a Thunderbolt display or a MacBook Pro as an external monitor?

Thanks very much


Julian