-50% HP is doesn't mean losing half your HP, as stupid as that might sound.
The -50% penalty is added to all the other modifiers you might have, which are Picture of Health, Lone Wolf and the difficulty modifier. The difficulty modifier is +60% on easy, +15% on normal and -25% on hard.
So without other talents, Glass Cannon would change the HP like this
easy no PoH, no LW: 160 -> 110 = -31%
easy PoH 2, no LW: 170 -> 120 = -29%
easy PoH 3, no LW: 175 -> 125 = -29%
easy PoH 4, no LW: 180 -> 130 = -28%
easy PoH 5, no LW: 185 -> 135 = -27%
easy no PoH, LW: 230 -> 180 = -22%
easy PoH 2, LW: 240 -> 190 = -21%
easy PoH 3, LW: 245 -> 195 = -20%
easy PoH 4, LW: 250 -> 200 = -20%
easy PoH 5, LW: 255 -> 205 = -20%
normal no PoH, no LW: 115 -> 65 = -43%
normal PoH 2, no LW: 125 -> 75 = -40%
normal PoH 3, no LW: 130 -> 80 = -38%
normal PoH 4, no LW: 135 -> 85 = -37%
normal PoH 5, no LW: 140 -> 90 = -36%
normal no PoH, LW: 185 -> 135 = -27%
normal PoH 2, LW: 195 -> 145 = -26%
normal PoH 3, LW: 200 -> 150 = -25%
normal PoH 4, LW: 205 -> 155 = -24%
normal PoH 5, LW: 210 -> 160 = -24%
hard no PoH, no LW: 75 -> 25 = -67%
hard PoH 2, no LW: 85 -> 35 = -59%
hard PoH 3, no LW: 90 -> 40 = -56%
hard PoH 4, no LW: 95 -> 45 = -53%
hard PoH 5, no LW: 100 -> 50 = -50%
hard no PoH, LW: 145 -> 95 = -34%
hard PoH 2, LW: 155 -> 105 = -32%
hard PoH 3, LW: 160 -> 110 = -31%
hard PoH 4, LW: 165 -> 115 = -30%
hard PoH 5, LW: 170 -> 120 = -29%