440c tempering question

So I've recently started experimenting with stainless steel and i figured that 440c would be a good place to start. I have made and heat treated several knives and felt like i was getting good results so I brought one of my knives to my friendly local machine shop to test the hardness. We checked the knife in 3 places and it came up 61.5 rockwell C consistently. I tempered at 320 degrees for 2 hours 1 time(after letting them warm up to room temperature from a dry ice and kerosene bath). I was thinking at the time that I would rather have them on the hard side so I could see what is possible with my home equipment (I'm not a professional by a long shot) but 61.5 seems too hard for a 440c blade. I heat treated these knives about a month ago so I don't know if I can put them back in the kitchen oven for another tempering cycle at this point. I would just try one at say, 375 for 2 hours and bring it back to the machine shop to test but it's definitely time and money to go out there for another hardness test so if I'm guaranteed failure I'd rather not bother. Thank you in advance for any incite.

Update: I have been out in the shed chopping copper wire and hammering this knife through plywood (a rip cut against the grain) and it didn't chip at all. I did a brass rod test and put my full weight on a fine edge and it bent before chipping. I don't know if the rockwell tester at the machine shop could have been out of calibration or something like that cause these blades are acting like my benchmade barrage (154cm) when put to the same tests.

I have my 440C done at 60-61 by Paul Bos at buck knives and have never had or gotten reports back from customers about the blades chipping out.

I make mostly Culinary but even my B & T knives that are a modified version of my kitchen 5' utility knives have passed with flying colors.

For anything Bowie or camp chopper size I have had the 440C taken back to 57-58 without any problems. I wouldn't think there is anything wrong with the tester you are using?

SM Chris I do two temper cycles and end up at 60 but think you should be fine at 61, though may have some retained austenite from just one temper cycle and wear resistance may be very slightly reduced but toughness is higher. I like 440C and think is a great steel though it is not trendy as the CMP's, for instance.

Thank you both for your incite. I was definitely worried that there was something I didn't account for. I will have to learn more about retained austenite as my understanding of it was totally different. I thought that it was the tendency for the structure of the steel to convert to austenite despite the quench (so austenite could form through the ttt curve and even at room temperature). I built a set of water cooled quench plates and as soon as the blades reach ambient temperature I place them in dry ice and kerosene to prevent them from doing that. I know that it's impossible to have 0% retained austenite but if conditions are optimal it should be as low as possible. I thought the point of tempering was to heat the steel enough to allow the structure of the steel to move around enough that the built up stress from the quench could be alleviated, thus reducing the tendency of the grain to be fighting itself. I know I need to learn a lot about the metallurgy and I am very interested in it but i don't know at what point my understanding is flawed and finding good information that doesn't confuse the hell out of me is not easy.