Myself, i have zero experience with the mountain rifle. I do have experience firing hundreds and hundreds of shots through the original Accura, and also the new version. If you are going to use BH209, it would be best for you to either modify the breech plug, or purchase the made for BH209 breech plug. If you use the original breech plug, one day when you least wish for it to happen you will probably get a hang fire. You could probably start with what ever bullet you used in the 700ml. A start load of BH209 could be 85g. See how things go from there. If you want bullet advice, then start out trying the 240g XTP. Using this bullet increase the powder 5g or so at a time, if you wish, until about 120g.
It may be not everybody will agree, but i believe it is essential to change the breech plug if you wish to shoot BH209. Please be aware, i have never held nor seen the mountain rifle; i presume the breech plug is the same as the Accura V2.