There isn't an actual pass mark for the 11+ in Essex/Southend - and the rules are slightly different too, depending upon which Local Authority area you live in.
Since the pass/fail mark was done away with several years ago, the school try to assess a standard as a notional pass. They then allocate a number of spaces for girls from the Southend LA area and the rest are available to everyone else. This generally means that the marks you need to achive to get a place (if you live outside of Southend) will be a little higher. From the experience of others (which you will find all over this forum) if your DD can achieve marks of around 70 - 75% across the board, then she should be OK. However, as it is all relative, and depends upon how well all of the girls have done in any particular year, there are no guarantees at all. This all helps to add to the pressure for us mums and dads, which we then work to protect our DCs from!
However, my personal view is that it is a little early for you to focus on the actual marks bit, but firstly to understand whether grammar is the right choice for your DD in the first place. If she is doing well so far, then that is great, but if you will have to push her hard to do well in the 11+, then you will need to consider how she would then handle the grammar school experience in its entirity. Perhaps a chat with her primary school teacher would help with this.
If she is doing well, then go for it! Our DD started at WHSG this term and is loving every minute so far - even the homework!
Hope this helps