define a background to be used for threshold

[gabirose]

Hi,
Need help with a macro, would appreciate it.
The aim is a semi automatic particle analysis using a macro.
Here are the necessary stages:
1. Manual selection by the user of a region with no particles. 'Measure' to get mean intensity in this area.
2. Take this mean and multiply it by 2.
3. The mean x 2 is a value needed to set a lower threshold of the image.

Currently my macro can do stage 1 but how can it pass to the next stage?
Thank you.
Gabi