How are her HP gains calculated?
Since her constitution is 17 her maximum HP gain, if she levels up in both classes, should be 4 + 1 (druid) and 5 + 1,5 (fighter).
Now, does that half hit point always become rounded down for each level or does it get stored to become a full HP during the next level-up?
I assume the former is correct?
Jaheira's HP gains
At level 1/1 Jaheira should have 12HP I don't think the .5 holds over I think it rounds up every time so as she advances she would get 5 per druid level and 7 per fighter level which would be 108 at level 9/9 and then after that 1 per druid level and 2 per fighter level.