There’s a way to make T-like meeting pipes of equal diameter. If that is what you need do like following. Create a tube. Select the vertices of the half tube , I mean not one base , but by length of the tube. Then SHIFT-K and choose Exact line from the menu. Cut oblically under 45 degrees your tube. Use the diagonal of the squares of the grid as a help. Do this once more , but this time under 45 degrees from the other side , that is 90 degrees relative to the cut made before. Deselect all(AKEY). Select only the vertices made by cut. Press YKEY to split the piece of mesh and then XKEY-Vertices to delete that piece. Select again the vertices made by cut but remaining after delete , and extrude them perpendicular to the tube. You get the new tube , attached to the old one. To make it clean , use once more the Knife tool to cut that V-like end of the new tube.