What does everyone use to equalise the weight of blades? My replacement blades .for the PT25 differ by about a gram. Is it critical that they all weigh the same?

Once pairs of blades are balanced on a pivot, it is unlikely they will weigh the same, even if cut from the same sheet as the density varies so much. It is the moment that's important which is mass x distance from the pivot. The should be about the same weight, but that can be adjusted with coats of paint, dope, or sanding to make them lighter.

Once they are about the same weight, pairs can be bolted together at the mounting hole and placed on a pivot, as shown in Rich's photo. Ideally they should be bolted to a plate which has a pivot in the middle, and holes for mounting the blades at the same radius as the holes in the autogyro's head. That way the moment of the blade is correct.

Next, the lighter blade can have varnish or tape added until it is balanced against the other blade.

You probably didn't want all this, but the balancing can be done with paint., dope, extra covering or tapes.

To illustrate that weight doesn't matter see

Single Rotor Blade Helicopter. (Experiment) (8 min 32 sec)