If you have not already solved your problem here are a few suggestions.
If you add a measured amount of slurry to a measured amount of hot water so that all the ice melts, then the change in temperature of the hot water can be used to deduce the amount of latent heat used in melting the ice, and hence the amount of ice.
Comparing the density of the slurry with that of water 1.0 g per cc and ice 0.92 g per cc with the density of the slurry should also give you % of ice, but it might be difficult to measure the volume of the slurry.if it is composed of large lumps of ice.
You could measure the weight of the slurry then drain the water from the slurry though a cloth and measure the weight of the water and the change in weight of the cloth.
Using all three methods you could determine which is best and most convenient.