Things to consider:

New or old lens and lens material.

CR-39 is the most versatile material to tint. Not much to worry about. Check old lens for defects and any coatings (ex. AR) applied. AR may need to be stripped first, this could damage the lens.

Polycarbonate can be tinted if a tintable scratch resistant coat is applied. With poly it is tougher to get a very dark tint, more so in gradient. Figure on a darkness of 70% to be the darkest.Poly is very difficult to have the tint removed or lightened. Try to use a few ounces of dish detergent in a hot water bath, about 200 degrees F. Old poly, try at your own risk!

Low index plastics, 1.53 does not appear to be any different than CR-39.

Mid to High index is similar to CR-39, sometime scratch coating can craze on dark tints, this is due to the time and heat applied. 1.67 material is the worst in this category. Again, beware of added coatings.