I'm not a tank but i do know your math is wrong there, when you add another percentage it doesn't add like 50+20=70.
Sometimes you do add percentages but sometimes you don't. It all depends on what mechanism is specifically being buffed.
In the case described, there are actually 4 attributes you have to pay attention to: the Guardian Leap DR buff, the Guard damage transfer, the Taunt damage debuff, and the Guardian Leap target's existing DR. Guardian Leap's DR buff is added to the target's existing DR rather than multiplying through because it's a flat buff to an existing attribute. As such, the 20-30% DR that the target has would get brought up to 40-50% DR for the 6 sec duration. The other attributes (the damage transfer and the damage debuff) would need to be factored in multiplicatively because they're separate mechanisms. As such, assuming the healer referred to is a Scoundrel (25% DR), the sum of those buffs would increase functional DR to 80.75% (1 - (1 - (.25 + .2)) * (1-.5) * (1 - .3)).
Keep in mind that, of those 3 mechanics, only 1 on them is Guardian specific. Without it, functional DR would be 73.75%. That's still an improvement of taking 26% less damage, but it's nowhere near the monumental rewards inferred to when it was brought up, not to mention that it's only up for 6 seconds every 20 seconds whereas the other tank classes simply have 5% damage debuffs that are easily applied and functionally permanent. As such, Guardian Leap is largely a break even with the damage debuffs that the other tank classes get. The only real advantage provided is the mobility gained from ally leaping.