This probably is not necessary. Aside from that, currently it appears that it is way to small to even bother implementing for the suggested change. Some other cards use with Catapult:
For Armagio, ((25*100)/(25+100)), or 2500/125, or 20 is the norm. With new formula, ((25*100)/(25+100-1)), or 2500/124, or 20.16, round to 21. Gains 1.
For Colossal Dragon, ((15*100)/(15+100), or 1500/115, or 13.04, rounded to 14 is the norm. With new formula, ((15*100)/(15+100-7)), or 1500/108, or 13.89, also rounded to 14. No change.
For Elite Armagio, ((30*100)/(30+100)), or 3000/130, or 23.08, rounded to 24 is the norm. With new formula, ((30+100)/(30+100-2)), or 3000/128, or 23.44, rounded to 24. No change.
For Massive Dragon, ((30*100)/(30+100), or 3000/130, or 23.08, rounded to 24 is the norm (same as Elite Armagio). With new formula, ((30*100)/(30+100-8), or 3000/122, or 24.59, rounded to 25. Gains 1.
For an extreme case:
For Crimson Dragon + Basilisk Blood, ((23*100)/(23+100)), or 2300/123, or 18.70, rounded to 19 is the norm. With new formula, ((23*100)/(23+100-12)), or 2300/111, or 20.72, rounded to 21. Gain of 2.
For Ruby Dragon + Basilisk Blood, ((22*100)/(22+100), or 2200/122, or 18.03, rounded to 19 is the norm. With new formula, ((22*100)/(22+100-15)), or 2200/107, or 20.56, rounded to 21. Gain of 2.
Results show that the change is small to nothing. The formula may need to be more than just subtracting the attack.