To be conservative, I would use designday wetbulb temperature to prevent the cooling tower from being over-sized in the baseline case.

I think your reset strategy looks good if you used a calculated approach - which means X would be replaced with 9.2°F. If you kept the approach constant (0F), there would likely be a net penalty because the increased cooling tower fan energy would negate any saving seen from reducing the lift across the chiller's compressor. 9.2°F.