Sorry to bother you again....
BlackHornet: I've tried to integrate your rather excellent code into what I'd previously written.
I have done something wrong as my results aren't the same.
the calculations start off well, but something go amiss when the two different colours are combined.
Would you please have a look at the linky here:
https://www.dropbox.com/s/dyaf8ymhl3m5vow/WontWork.capx?dl=0
...and tell me where I've gone wrong?
Many thanks.