Assuming the next level is 1/2 the buyin, I would move down once you have dropped 20 buyins and therefore have 60 buyins at the next level. Move back up when you are topped up to 50 buyins.
Thanks, I think this is a better approach than what I've been doing. I've been allowing my roll to drop a bit too far and it seems like it is going to take forever to rebuild. I grudge earning less $/h and so hate the idea of moving down stakes but I think it's time to change that attitude and be far more protective of my roll.