initial memory pool and maximum memory pool in Tomcat 6.x

[Tuan Quan]

Dear all;
What is the advantage of setting both values to the same number?
My application run out Permgen memory,
If I set both memory pool to 1200MB, and Permgen - 256M, Tomcat will not start.
but if memory pool to 900MB and Permgen to 256M, Tomcat start OK.

thanks.