Hmm - found 2 ways of being 2 off lol - I can fill the last 2 holes with either of the last 2 blocks first - both end up 2 short. Now I've just gotta figure out which one is correct and try and optimise it .... EDIT: Done ....
I use this trick often in harder levels. It really only trips up the most advanced players who do a one-way lock instinctively and never look back. If it's well concealed, newer players will usually spot it first, haha
Insights