I’m not sure the majority of memory requirements are from Spring Boot but from the additional amount required running a Docker image in PCF. This post from a while back talks about Spring Boot memory usage. https://spring.io/blog/2015/12/10/spring-boot-memory-performance
From my experience with PCF, I’ve had to increase the memory size about 500 mb just for the Java 8 base image. After that the Xmx I need can be tuned and just added to the base 500 mb.