I am wanting to know the formula of calculating the percentage of a value between two numbers, for example -65dBm is 100% signal and -93dBm is 0% signal.
I'd like to know the formula of a value between those two numbers, example: I need to know what the percentage will be if the signal level is at 70dBm.
Here are two suggestions to show you why your question is not well defined per se. I have suggested two possible curves to model data points between the known extremes at $(65,100)$ and $(93,0)$. Which curve do you prefer and why?