I got one of these to do the job. I marked the measurement. But a packer makes sense as I was struggling to keep it on mark while cutting.Buy 100 plastic box for s shape and and find something 8mm to pack the bottom so cove sits correctly. So the base is 92mm but stands 82mm high in block if correctly packed with 8mm packer. Size is written on the back of coving anyway but I think I'm right... I use block like picture and powdered cove adhesive plus always dab Pva or something on cut edges so it done shrink or crack. I always put adhesive in joints so it oozes out. Last lot of cove adhesive was from wickes but hard to find it, but any fast set works like easyfill 20. Also blocks on net for coving that are adjustable. I'll go look later for links
Nice tip. Sometimes the obvious it's overlooked. Ta
PVA to cut edges is something that I have done before but kinda thought I was being over the top. And neglected it on last couple of rooms. They did only have internal cnrs.
Will have to pull myself back into line.