I'm not sure if this is right and I'm sure someone will correct me if I'm wrong, but I would use one of the inner 'border' or strip lines to measure against. So, if for example the strip on the right was cut at 2 1/2" and joined to the rest of the block, I'd be laying my ruler along the sewn side and measuring 2 1/4" (1/4" off for seam you've just made) from there.
This is how I was taught to square quilts etc - by using the inner border as my guide.