A tank doing less than 750DPS - guard the best damage dealer in the group.
Most tanks, unless they're in absolute BiS and know their rotations *exceptionally* well aren't going to be dealing 750 DPS, especially not Guardians. You only arrived at this arbitrarily high number because you actually did most of your math wrong.
First off, it should be noted that the high threat multiplier is multiplicative: it generates 50% more threat *before* the tank stance doubling occurs. Ergo, a high threat ability generates 300% threat and all of the other abilities generate 200% threat. In addition, every tank has some form of high threat ability: the Ion Cell DoT for VGs, Guardian Slash for Guardians, and Slow Time and Force Breach for Shadows.
Secondly, it's also not required to maintain an exact doubling of threat in order to maintain aggro. For a *vast* majority of cases it requires 130% of given threat to pull threat because the "ranged" range for threat purposes is measured as 4m from the center of the target whereas the attack melee/range range is measured as distance from the outside edge of the target's model. As such, a majority of melee DPS actually end up DPSing within the "ranged" threat radius and require 130% of current target's threat to pull.
As such, in order to pull maintain threat going up against a target generating 1800 DPS/TPS, a tank needs only generate 1385 TPS without ever having to taunt (which is ludicrously simple even in bad gear).
A healer doing more than 3000HPS - guard the healer.
It's been mentioned already, but I think it bears repeating, you're only correct in this sense if and when you're dealing with a single enemy NPC. In all other situations, it would require substantial geometric growth of HPS in order to threaten to pull aggro because healing threat is diluted by the number of targets currently in combat (and this is completely ignoring the fact that all of the healers have and should take the talents that reduce their threat by either 15% for Sages or 10% for Commando Scoundrel, meaning that even more HPS is required): with 2 NPCs in combat and a tank generating 1500 raw TPS (1950 TPS required to pull), a healer would have to generate 8667 HPS (1950 / .9 / .5 * 2), 3 targets would require 13000, etc. Even with a single target, the HPS is so high as to be functionally impossible to reach (4333 HPS).
As such, the only times that a healer even stands the *remotest* chance of having aggro are at the very beginning when a target has taken no damage whatsoever and any degree of threat reduction will serve absolutely no purpose whatsoever or at the extreme end of a fight wherein a target that has been lightly brushed by a single attack and has been ignored otherwise for an extended period of time *finally* has enough threat gained via diluted healing threat to pull aggro wherein the threat reduction *will* actually lengthen the period of time before this occurs (keep in mind, a healer generating 1000 HPS would take 9 seconds to pull threat on a target hit for 1500 threat once with 3 other NPCs in combat; more enemies, more damage, or less healing required are all going to push that number even higher; Guard would simply increase that to 12 seconds). In both of these cases, the threat pulling is predicated upon both of the DPS and the tank ignoring a target for an extended period of time, which just really isn't going to happen unless the group is absolutely *terrible*.
The only real benefit that a healer gets is the 5% damage reduction, which isn't even additive DR (additive DR is added to your K/E or I/E damage reduction and, as such, is more powerful; multiplicative DR is simply a percent reduction in all damage taken and is factored in post mitigation) and really shouldn't even make a difference since the healer shouldn't be targeted in the first place in order to *need* to benefit from the DR. Unless everyone else in the group is a complete idiot *and* the healer is vastly overgeared, the healer should never be under the risk of having threat and needing the assistance of a Guard.