If you can get 225*, go for it!!
I have a hard time keeping below 250* in my kettle, so thats why I suggested the higher amounts.
You will need to add a couple "lit" coals during the cook no mater what to maintain your temp range.
Pyrex probe thermometers are $15 at Meijer.
Sometimes people will even run hot water in their cooler, to "pre warm" it, but this shouldn't be necessary. The point is to just bring down the temps slowly. I leave the probe in there, and keep the thermometer on the top of the cooler. When I see 160-165*, I know its cool enough to slice without burning my fingers!!
"Everyone has a plan until they get punched in the mouth." - Jack Reacher
Bandera, M-BGE, WSM, Gold Kettle, Kenmore Gasser, 36Qt Turkey Fryer, ECB, Dutch Oven