Calculator

This should be easy but I am havining a mental blank.  I have four fields each with a formula

Total   -  Total of 1 2 3

Item 1 - 50% of Total

Item 2 - 40% of Total

Item 3 - 30% of Total

Now most of the time the user will be punching in the Total so the other calcs will work, but occasionaly this need to work backwarrds,  ie I need to punch an amount in Item 2 and all the other fields fill out.  I am going round in circles with this one.  I keep getting circular references continually.  Anyone have any bright thoughts I am missing  ;-)

Thanks