How do I use my power meter file to improve my time up this steep hill climb tt? The hill is 0.8km long with an average gradient of 13%. My cadence on my 46t-36t 11-25t 9 speed gear ratio starts at 80rpm and rapidly dips to 40 rpm. Is there a way to calculate what gear ratio is best for me to move to?
Aside from pacing better, if your cadence drops to 40rpm for too long and you are in your lowest gear at maximal effort for the expected duration, then you could probably use at least a gear 2/3rds or less than you currently have so you are likely to be able to sustain closer to 60rpm. 

That's possibly not feasible on your bike, but perhaps a 29 rear cog at least would help get cadence up a bit higher. Being stuck at 40rpm is probably (but not certainly) compromising your ability to generate maximal power for the duration. Depends on how often or how crucial this climb is to your overall performance goals. If it's not that important and you only deal with such climbs rarely, then making such a gear change probably isn't worth it.

Of course if you are carrying extra dead weight, then working to trim down can also help since steep climbing speed is all but linear with your power to weight ratio. At least a power meter can provide additional insight into your overall metabolic energy demand from training so that you can manage diet and medium term energy balance for sustainable/sensible weight loss.