Magic mitre block has a stop which will fit in 3 locations so will cover 100mm. 127mm and 135mm coving made by British Gypsum.
MAGIC MITRE BOX. In 3 simpele stappen zaag je een perfect verstek, je kunt de Magic Mitre gebruiken voor een groot aantal uiteenlopende producten zoals In drei einfachen Schritten sägen Sie eine perfekte Gehrung.
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Or buy a block that's 100mm or more across. I use 100mm one and place something in that (wood block) 8mm on the base allowing a distance of 92mm across for your 135mm cove. That means the cove will sit correctly and cut fine. Coving sits upside down so the ceiling edge is the floor edge when you cut. If you get it sat in box correct the virtual should be 82mm for 135mm as odd. I think.. I'd need to check my measurements but written on the back of cove, its been a while so foggy memory.
Don't get asked for it these days
One of these and pack base edge with 8mm block of wood leaving the 92mm for cove.
Some just use 127 block but it's wrong.
Wonder mitre is wrong. BG made a sculptured one similar but never seen one for sale. I phoned them but no joy. Clueless