Normal Attack: 36-39 dmg (based on 46 results)
Normal Attack Criticals: 54-59 (based on 48 results)
36 * 1.5 = 54
39 * 1.5 = 58.5 ... ~59

Bootshine (130 Potency): 61-68 (based on 21 results)
Bootshine Normal Criticals: 92-101 (based on 27 results)
61 * 1.5 = 91.5 ... ~92
68 * 1.5 = 102

Bootshine Opo-Opo Form Criticals: 97-126 (based on 20 results)
61 * 1.07 (greased lightning bonus) = 65.27 * 1.5 = 97.9 ... ~98
68 * 1.07 = 72.76 * 1.5 = 109.14 ... ~109
68 * 1.07 * 1.1 (twin snakes bonus) * 1.5 = ~120


Basically, don't forget bonuses from other skills in your calculations. The only significant difference is in the max Bootshine Opo-Opo crit that you got, but I'm guessing you just got lucky with a nice base damage (about a 71 normal bootshine), or you had some other buff as well.

End result: The crit multiplier is 1.5.