Fibrotic index

[raiheu]

Dear Listers

I am new to ImageJ and would like to use the software to evaluate the fibrotic index in tissue samples from chronic pancreatitis and pancreatic ductal adenocarcinomas after SiriusRed staining.
Looking at pancreatic ductal adenocarcinomas (see attached photo), there are different areas within a section:
1 : collage fibers (pink, SiriusRed), which should be interpreted as area
2 : remaining ductal tissue and islets
3 : ductal lumen
The fibrotic index should be calculated from: 1/(1 plus 2 minus 3)
My question:
Are there any existing plugins, macros or even detailed step-by-step description for this purpose?