30 watts is the maximum that the chipset can have with the minimum resistance recommended by evolv, ( 0.25 ohm )
The higher the resistance, the less the current you will have. All value under this max current can be regulated.
The concept is explained perfectly by Mark Todd on his review on Youtube
Your knowledge and understanding of how to use a mod and rta above should be corrected. If your wish is to have 25w adapted your resistance to this calculation (this should be around 0.4ohm). The facebook group with hundreds of people using this mod can help you too