watering food plots

[wmg]

My plan next year is to make a watering wagon (500 gallon).  I've seen several youtube videos of "mobile firefighting rigs" and already have most of the supplies for that.  does anyone have a recommendation/advice on how to determine how much water needs to be applied.  I understand I will have to consider what i plant and type of soil, what I am looking for is jumping off point such as 200 gallons/ac because someone knows this equals 1/4 of rain, or " My grandpa used to say mud between your toes makes your crop grows"  - I know this will be trial and error, just try to reduce errors when i can