Even if you do it yourself, you should consider the thickness.
Here's a formula you can use:
Square feet x thickness (mils) / 1604 = gallons needed (at 100% solids, no solvents)
The solvents evaporate away, only the solids stay on your Jeep. So, if the product you're using contains solvents, you need to bump up the gallons needed to compensate.
For example, let's say you measure 200 square feet and you want 70 mils thickness. (For reference, the thickness of a dime is about 53 mils.)
So, 200 x 70 / 1604 = 8.73 gallons
But, the product is 30% solvents, so you should bump up the total by 30%, so 8.73 x 30% = 2.62 gallons. The total needed now is 8.73 + 2.62 = 11.35 gallons. But, don't forget about waste such as the amount of product stuck to the roller! So, 12 gallons would be appropriate.
Professional jobs should be about 70 to 100 mils.