ive put some windows into a regular brick built building, But when i have pointed the window horns the pointing has cracked where is meets the original brickwork once dried. i fitted a few windows over a period of time so have experimented a bit with different ratios like 3 parts sand to 1 part cement and then 4 parts sand to 1 part cement.  I also tried wetting the bricks before trowling the mortar in but it still cracked.   this isn't my image but it shows where the door sill horns e