Your idea is fine although you should place the blue tack on the tonearm counterweight. Then balance the tonearm. Removing the blue tack would then give you the 1.7 grams.
Is there a problem with setting VTF this way?
I was wondering if anyone else has tried setting their tracking force this way. I have a very accurate laboratory beam balance scale with which I weighed out a blob of blue tack weighing 1.7 grams. I attached this on top of the headshell over the stylus contact point. I then adjusted the VTF until the tonearm was balanced, horizontally level in equilibrium. I removed the blue tack, VTF should be set at 1.7 grams. Is there a problem with this method I haven't thought of?