If you are looking to be in the top 33%, I would take a look at last year's results to see approximately what time you would need.

As all race courses are different Longislandguy probably had the best advice - look at the race results from last year for the race you are doing.  You may have to be a little bit faster than last year but at least you'll have a good idea of what you need to do for your race.  Additionally, what length race has a half mile swim?  Typically a sprint is about a third of a mile, a Olympic swim is just under a mile, and a IM 70.3 is just over a mile.  And...take it from a good swimmer....people spend too much time worrying about the swim when they should be out running.  Good luck and have fun.

I TOTALLY agree with dk.

I am fairly new to triathlon, doing 4 sprints last year.  I completed my first sprint this season a week ago at the Minutman Triathlon.  I have an Olympic distance race coming up in a little less than 2 weeks.

Swimming is all about finding a pace that works for you and not worrying about speed.  I have been pool swimming my entire life and consider myself a very solid swimmer but that all changed my first few open water triathlon swims.  Anxiety and nerves kicked in as I was too busy trying to find a spot and keep up.  I was exhausted in transition and took a long time to get my wind back on the bike.

It is important to note, I am physically challenged, born with achondroplasia, a form of disproportionate short-stature, more commonly called, dwarfism.  Prior to the start of this season I had time to do 2 open water swims with my local tri team here in eastern MA.

When I started the sprint last week, I found my spot, got in the water, and swam.  No anxiety or nerves and a ton of fun.  I got passed by many, and passed a few.  I got out of the water and had lots of energy to have a great T1 and get on my bike and GO!!

I hope this info helps.

"Triathlon has become life in microcosm, a metaphor that gives truth to  the wisdom passed from each generation to the next: work hard and you  will be rewarded, have faith in yourself and you will excel; do not  falter when an ill wind blows your way." Ashley Halsey

DK has great points for total time and robbing time down the road to gain a minute up front.

Last year I focused on the bike portion, because it is the longest, using similar logic.  (And because it kicked my butt the year before).  You'll spend about half of the Tri on the bike.  The biggest proportional gains will be made there.  My last sprint I swam the swim medium fast, focusing on being fresh out of the water.  Then I hammered the bike leg, taking 10 minutes off my time from the year before, and beating even my best training pace by a couple minutes.  It cost me, though, and my 5k was slow... about 2-3 minutes longer than expected (still over a minute up on the year before).  However, that gain on the bike was a good trade-off for me.

It sounds like you've got the right idea, though.  The swimming should come together for you in the next month, and you'll be knocking out the distance in no time!