Store floats in and reading them from flash
There is a helper macro in avr-libc that is designed just for this purpose:

#define pgm_read_float_near(address_short) __LPM_float((uint16_t)(address_short))

Read a float from the program space with a 16-bit (near) address.

Note
    The address is a byte address. The address is in the program space. 


Use it like:

PROGMEM float pi=3.141592653;  // constant value in flash
float diameter = 24.332154;    // variable user value

float circumference = diameter * pgm_read_float_near(&amp;pi);