See CoM p.51
"For Potions and Charms with an Attack Spell Effect, make an open-ended roll on the RR column of the maneuver table with a +50 bonus".
When this bonus isn't what i'm searching for, i use this rule, found on p.52 :
Many formula potions provide bonuses to skills and maneuvers. For each additional +10 bonus (after the 1st +10) per skill, add 20 points to the cost of the potion, e.g. a Potion of Jumping gives a +50 bonus that costs 80 points (+50 - 10 leaves +40, for 4 x 20).
For me, casting an attack spell with an open-ended roll on the RR column is a maneuver. So i use this rule to let my players create formula potions with attack spell (RR) +70, +100, etc.
Below means that in a formula potion, you create your effect with aspects and attributes, and after calculate the cost of each. This is described p.51 Formula Potion, and p.32.