Okay so the problem is that the median and average HP per level in Bestiary 1 is not a linear function. It grows slowly between -1 and 2, from 3 to 15 it is almost linear but slightly speeds up, and after 15 it slows down and even reverses (and generally behaves weirdly because of the small sample size). The effect of that is that if we use linear extrapolation, then troops based on low-level monsters will have too few hitpoints, and ones based on high-level monsters will have too many hitpoints.
Maybe the easiest way would be to introduce a special case: for base levels of -1 to 2, add base monster's HP to the formula result one more time. This would move the Zombie Squad to 160HP, Zombie Horde to 200 HP, Hobgoblin Soldier Squad to 93HP. I don't think that the high-level troops need adjustment just yet because I don't think there will be many troops where individual creatures are levels 11+ and there is not enough data to set a clear benchmark for high levels. But probably subtracting 1/2 of the base monster's HP won't throw the results too much off in those cases.