How do i work out how much material to order?

Hi there

It's been doing my head in for ages!
How do i work out, on excel, how much material i need to order to make some curtains?
I have attached my (lastest) spread sheet. I've got it to work out the sizes and so forth but i can't for the life of me figure out how to get it to work this out?
HELP!!!!

Many thanks

jad007

=MROUND((((C10*1.75)/2)+4),13.5)/54

Hi there
C10 is the width in inches and is multiplied by the gather of 1.75; divide by 2 because there are 2 curtains to a window then plus 4" for the hems. Mround to a multiple of 13.5 which is a 1/4 of 54" (the standard width of curtain material) and we only cut within a quarter of the total width (e.g. 13.5, 27, 40.5 or 54. 1 1/4 will comprise of a 54" and a 13.5" sewn together). Then divided by 54, to give the fraction.

You can see why it took me ages! This is very good for my brain power!!!!

Hope you can help

jad007