I don't think you need to have the exact temperature, just approximate and it'll be fine.
The best way to know for sure is a thermometer, all other methods aren't as reliable. However it's not worth it using one each time you brew.
By the way, when I boil water it only reaches 91.5 C (196.7 F), does that mean that I never get to try the real taste of genmaicha because the temperature is too low?
Made from leaves that have not been oxidized.