I have a 600 foot grass runway, 800 feet elevation, next to my house which I flew my Brako Sprint with a 912 ULS engine. Both ends are fields so there is no issue with trees to clear. Using the standard 200 RRPM, stick back and full power I used almost all of the runway to get airborne. Watching my videos it was clear that getting the rotor up to flying speed was taking the longest time. My RRPM needed to get to 280 to lift me off the field in ground effect, and would fly at 340 or so. Trying different procedures and evaluating the videos, I settled on a starting RRPM of 260, stick full forward, advance quickly to full throttle and pull the stick back after 2 seconds. Holding the stick forward for two seconds minimized the rotor slowdown versus having the stick back. Having the stick back quicker or at the start of adding full power slowed the machine acceleration because of the drag of the rotor and increased the slowing of the rotor before there was enough ground speed to begin accelerating the rotor. This procedure gets me in the air around 350 feet so plenty of room to abort if something is not right.
As for flying with your instructor I would suggest you both wear swimwear because I do not think there is any way you will get off, two up, in 250 meters of grass!
RandyV