Tracking preregulator control

[gigabyte091]

Hi, as a part of my diy variable PSU i bought dc-dc stepdown converter from ebay to act as switching preregulator.

Everything is working fine but now i need to set switching regulator output about 3-4V higher than linear postregulator output.

At the moment, voltage of switching converter is set by multiturn trimmer with resistance of 10k ohms.