i also think you should have more ,
i don t think that calculator is very accurate . my smaller oven has about 9000 watts and takes around 12 minutes to heat up .
also your inner walls being 16 gauge it will take longer then some other home made ovens that use 18 or 20 gauge steel. you have more mass to heat up and takes more time for heat to sink in 16 gauge steel , however being thicker once its heated up when you open the door to put parts in the oven should reheat faster because ovens with a thicker gauge stay hot longer
