Yes, I've installed the module "Linear / Perimeter / Surface (Area) / Volume / Weight Calculation Module" which handles the selling by weight issue pretty well but it seems it isn't possible to accurately manage stock quantities when using decimals.
It seems strange it is so hard to do what I thought would be a common requirement of selling a product by weight and keeping accurate inventory. It is also made harder by the fact the grain has attributes such as 'separate bag' or 'all together' which creates multiple combinations with multiple inaccurate quantities which makes inventory control a nightmare.
It would be nice if you could add a product option that didn't create a new combination. e.g. I have 5 x t-shirts in stock. If requested, I'll draw a picture on it before I ship it. Whether I draw a picture on it or not I still only have 5 t-shirts in stock and if a customer orders 1 x t-shirt with product option to draw a picture on it, total stock is now 4. However PS/TB would say I now have 4 t-shirts with pictures left AND 5 t-shirts without pictures.
I have read that maybe Magento, Zen Cart or Woocommerce might be able to handle product by weight with decimals but I really don't want to learn a new system after using PS/TB for so long.