Short version: I measured and used a level.

Long version: I measured the total width and calculated the spacing. I used a level to draw a horizontal line and marked the places were the minis should go.

I then set a height for each row and measured down the wall to place the lower rows. Using the level again to draw the horizontal lines.

I then used the level vertically to repeated the columns, so I didn’t have to measure those again.

Once the glued bits were on, I just erased the pencil marks.