The Dalokohs Bar - HP Buff calculation and mechanics
Solution 1:
The HP Boost does not stack with the medic-gun.
It heals for 60 HP, and bumps the max HP by 50. At the end of the 30 seconds, the extra 50 HP disappear immediately. If you are above 300 HP at the time, you are reduced to 300 HP (assuming no over-heal). Otherwise, there is no change for your current HP.
If you are currently buffed to 350 by the bar, a health-pack or resupply cabinet will heal you up to 350 HP. However, it appears that dispensers will only heal you back to the original 300.
There is more detail at the official wiki.