HI KEV HAVING DONE A BIT OF RESERCH
found this Laser Gradient Spirit Level 600mm
450mm (18"). Five horizontal vials with 0-4º gradient in 1º increments to allow for fall to drain water from roofs, guttering and driveways.
May be you need to learn a bit yourself considering you think you are an expert ha ha ha ha lolololol............
Its a good job im here to learn the greens from the yellows.
Just drill the flue hople a little above centre line of elbow and put a stone under the outside side. Stand back and say to yerself, would that send all the slops back to boiler? If using your many years of experience you consider it so, you've got your flue slop right.
With condensate drain, stand back and recon up, is that a goodly slop?
How come you need to be any more precise than that? Would have to be a very unusual installation.
Most common boilers come with a flue terminal and 800 mm flue length which when fitted should be horizontal because its designed with a slope on the inner tube already built in!
These is no further slope needed in addition to the inner one!
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