Subreport (in main report) position and height problem

Question

Currently i am developing Billing Application, in which i create windows form for Invoice, in this i use Crystal Report. in crystal report i use sub report for display Tax detail.

i use Sub Report in page footer section, number of Sub Report Row is not fixed.

so, at Run time Sub Report row is more than one then it's overlap with Main Report Text.  And i also attach screenshot with this post.