Although there is action all over the field, a lot of the play in lacrosse centers around the goal, to maybe 30 yards out. I find a few locations that work. Surprisingly, midfield will probably allow decent shots of both end with your 300. I shoot from there often, as there are a lot of plays where the offensive players will come from behind the net, or spin... allowing shots that capture their faces. If the 300 isn't quite reaching as far as you would want... just cheat 10 or 20 yards towards the goal you favor. There may be a little cropping, but that's not the end of the world. Or- you can get in the corner by a goal and get the action from just that end. 300 is probably a good length from there, but you'll have all the issues at various times... sometimes, they will be too close to you (try to shoot tight on faces then) and sometimes, they will be pretty far when the play is near the other sideline. Behind the goal line is good to, if you can get there.
All that being said... In a perfect world, I'd prefer something a little longer. Day games, the 200-400 is great, and I often use the built in teleconverter to push that a little longer. Night games, the 400 2.8 rocks.
Long story short... you can make that gear work.