On a blade that thick and it's profile I would start with 2 tempers @ 300°. You dont know the as quenched hardness and it could have been low depending on the temp you quenched at. I very rarely temper 15n20 over 350° in small blades. In larger items like a damascus sword with a high 15n20 content I will go much higher. But 15n20 likes to be ran hard. Generally 15n20 is not used for a chopper becaus of the thickness you can get it in. But even then I would not be resistant to try it at the harder side of things. Also your oven can have a very large temp swing and small blades can over heat. I temper all small blades clamped to a very large thick steel plate. It helps even out the temp swing and seams to prevent over heating.
it was in the oven at 375 once for 1.5 hours then 20 minutes on another cycle before k read this.