Half ironman power levels??

In addition to JK comments, you also have to consider your current cycling and running fitness. Those will give you a better idea to figure it out, the better fitness the higher you can go. Still you can test this with a few race rehearsals to make sure you don't leave your race on the bike

Yeah start high % on your trainign rides and go from there. You can do rides between 1:30-2:30hrs @ 85% FTP and run 3 Mi off the bike at open 10k to half marathon pace and if you can hold the pace comfortably you are around the ball park, you paced well, did a good job nutrition wise, etc. if you struggle it probably was a bit too much.

I am assuming you are training with a power meter; review your files from all diffeent rides (FTP, tempo, Long endurance, etc) and you can see the power trend and identify what your ideal power for a 70.3 is. Based on that do a few race rehearsals (one like 8 weeks out and one 6-4 weeks) and race at THAT intensity. Do all what you would do on race day: have dinner night before as you plan for the race, eat same breakfast, use nutrition plan, etc. Ride at the % of FT you think is ideal and run off the bike up to 6 miles at half marathon pace (faster than race pace; for a 70.3 your race pace should be around your open Marathon pace if in shape), again if pace feels relatively comfortable/doable then you you know your sweet spot.

Don't be affraid to do some 2-3hrs Tempo rides faster than race pace (i.e. 90% FTP) to grow stronger, you won't be racing at that intensity but then 80-85% FTP on race day will feel 'easier'