There is/are no standard formula/s.
You must first determine what you are calculating (value, cost of cure, cost of restoration etc.), then calculate it, then determine if it is reasonable. Cost of cure can be more than the value of the house and property while a single tree appraised by trunk formula should likely not be more than 5% of the real estate value.
The Guide for Plant Appraisal is the commonly used guide but the 10th edition is rumoured to be entirely different from the 9th and you should be farmiliar with the reasons for the change so that you can provide a arguement for your valuation method.