Hi! There is actually no exact way of calculating it percentage wise because different attacks will scale better with Nin over Tai and vice versa. This means that even though the attack is a Tai and Nin attack, the percentage of how much they effect the attack is different.

I usually will focus on Tai or Nin based on a number of factors including the growth rate (found in Characters > Detail) or the abilities of the character and whether the ninja's ability are mainly Taijutsu or Ninjutsu.

Hope that helps!

~Jib