Fold the triangle in half, being very careful not to stretch the bias edge, and finger press a crease at the center. Fold the piece that it is going to be attached to, in half and finger crease the center as well. Now match up the finger creases. Much better than just eyeballing it.
If you have a block that is a little 'off', it might be from too aggressive pressing. You can 'bring it back' by laying it on your ironing board and pinning down two sides. (Use a ruler to get them straight, gently tugging to line them up properly.) Now again using the ruler, measure and pin the other two sides to the size that they are supposed to be. It is ok if one side is too long, just ease it up to the length it should be and pin. Now wet it down with a spray bottle and leave it to dry. Very often bias seams that have had too rough handling will shrink back to their original dimensions.
Last edited by GingerK; 02-12-2019 at 02:53 PM.