Here is a tip.
I wish I could draw this but I can't so I will try to explain.
Every printing session we do this.
Example: 12x18 prints from 35mm negs= correct aspect ratio.
therefore we set the enlarger height to this size.
we set the easel blades to 12x18 full frame for this ratio of film
we place correct lens in holder ,
we set the condensers to the correct position.
we make sure the bulb is working.
We place a 35 negative carrier into place which is centered to the lens, condensor and bulb. we focus without a neg in place to see the outside edges of the neg carrier.( I tape with electrical tape the position on the 4x5 glass, I am not using a standard carrier and this may be where you are not seeing my workflow}
We then take the carrier out of the enlarger and project the light, we drop the bellows slightlyand therefore can see the condensor circles....
We then move the easel into position with the 4 corners of the blades touching the condesor circles.. this tells us we are exactly centered top to bottom.
We use Duct Tape,( because we love Red Green and are Canadians) and tape the easel into place.
We do not move the easel again for this printing session,,, If we need to crop we move the negative into place, remember I am using 4x5 glass carrier and can move the neg, it is not in a strip the negative is individually cut so we can do this.
I hope this makes sense. Moving the easel will cause some problems in some darkrooms, I have seen enough darkrooms to know this to be true.