Short Answer do not worry about pre bend, this is a Hobie 18. Jim posted up some nice previous discussions on this topic. After reading through them I got a head ache. I think they all ended up agreeing. Pre bend as it is typically thought of, does not really apply to the Hobie 18 due mainly to the sails (not cut or of a material that can handle mast bend fore and aft) and next is the mast itself, very thick section with bendy top section.
As mentioned above and in other threads, the spreaders do not allow for much in the way of mast pre-bend. The spreaders are adjustable if one wants to get carried away, however I think this is a great distraction. Pre-bend is usually referring to bend in the major (meaning fore and aft direction of the mast.) The rotating mast and particularly the Hobie 18 gets its mast bend mostly on the minor (meaning sideways bend). This is accomplished by mast rotation and then down haul and main sheet tension, which will bend the mast side ways and flatten sail. That is not "pre bend". The diamond wires on the mast are used to allow or limit the amount this sideways bend.
Now for the practical part, mast bend should be the least of the concerns for most (99%) of all Hobie 18 sailors. (I just sailed in a Hobie regatta this past weekend in the 18 class) and I will share my observations. First the Hobie 18 is a heavy catamaran and as mentioned in the threads under powered by today's standards. For class racing this means nothing, but I point it out to make a bigger picture point. We occasionally sail against a H18SX, bigger main, bigger jib with very little observable speed differences. So even "a powered up boat" results in not much difference being made in the speed of the boat. The boat's weight and hull design have some what dialed in speed potential. If one looks at the Portsmouth rating comparison between an SX and a stock Hobie 18 they are essentially the same even with the SX having a spinnaker. So all the mast shape worry is to miss the boat, so to speak. Now to my observations on the race course. First, I have no idea what my mast diamond tension is at, it is how it was when I bought the boat 2 years ago, (so I obviously do not think it is important, but after reading all of this I may check)
On the race course I notice a couple of things, one was my assumption that the wind direction and strength on different parts of the race course is and was key to boat speed. Not my mast bend or batten tension (don't even know what that was either). When we were right next to other 18 (like at the start) etc. we didn't point higher or go faster. However after we tacked and sailed off by ourselves to re-cross again with other 18's we had put 10 to 20 boat lengths on them. Some of this was by going the right way wind shifts and all, but what I think was the largest factor was they did not have a sense of their own boat speed. Meaning, they did not know if they were sailing fast when left by themselves. They would only go fast when next to another H18 so they could imitate their speed. My strategy quickly became, get away from any other boats and I would be faster. It worked flawlessly. This tells me the other boats needed more sailing time recognizing when they had the boat going at full speed, and when it slowed down when they are by themselves.
So back to the short answer. Mast pre-bend is a distraction. Read what you need to know about depowering the sail with mast rotation and down haul so you can focus on sailing. Steering the boat fast is everything, learn how to focus on that. Then look at where you have most wind, and which direction get you there fastest.
My disclaimer is, I am not trying to insult anyone. Just want folks to focus on the sailing part more as all the boat tuning stuff will distract folks way too much. The Hobie 18 does not lend itself to tuning changes making that much difference. If you do happen to be racing at the Nationals, it makes great dinner conversation, but again those guys are not winning because of their diamond tension.
PS I raced 18's hard in the early 80's, getting back into it and really love the 18 for its simple fun. I no longer need to worry about mast bend and batten tension