I'm going to sound a bit hyper cautious here but when you start going "off the charts" you need to be aware of all the risk factors. As reloaders I sometimes think we are a bit too concerned about pressures and ignore other factors that can effect safety.
From your picture I'm led to believe that you are working with powder coated bullets, which are definitely "off the chart" in every reloading manual I am aware of. My concern is the difference in the amount of friction developed when compared to lead or jacketed ammunition. Since not knowing this I would suggest that you make sure that you chronograph the very first round you fire using your new bullet. Because if your load produces velocity below what you expect it's a possible indicator of a higher level of friction being produced. With that knowledge you can then compare your result to that produced for cast, hard cast, or jacketed bullets in that weight. I would also suggest that you make sure to do a report on your results using powder coated bullets for all of our benefit.
The problem with higher friction bullets is twofold. One is the risk of a Squib with a starting load is increased. The second is that with some powders the higher level of friction may produce unwanted spikes in pressure.
So, how does this apply to you? First, if the friction generated is similar to a jacketed bullet your 3.6 grain load of HP-38 may be too light and you might leave a squib in the barrel. If you use the longer loading length the increased case volume will reduce the initial rate of pressure rise and thus may impeded ignition, further increasing the chance of a Squib. So, you really need to pay attention to the level of recoil and report volume produced and if you think there is the slightest risk of a squib check the barrel after each shot fired. BTW, I carry a Streamlight Stylus flashlight specifically because it's very easy to poke the light in the forcing cone and see if the light is shining out of the end of the barrel by pointing at the bench or piece of paper.
As for your overall length concerns, in a 38 special I don't see any real risk in loading 1/32 inch short. There is enough excess case volume with most powders in the 38 special than going a bit shorter will have an almost unmeasurable effect on pressure. As for going longer, as long as you are at the minimum recommended charge or higher for your bullet type I don't think there is much risk of a squib. However, I would personally bump the powder charge a touch if using a starting charge and loading long just to reduce the chance of a squib.
