With the advent of computer controlled kilns, producing crystal glazes at medium and high temperatures is quite easy, just use a really fluid zinc glaze and fire a schedule that gives them opportunity to grow.
Adjust the final temperature on step 3 to match the temperature your kiln reads when it reaches cone 6.
Start temperature assumed: 25°C or 75°F
"Fahrenheit degrees" is not the same as "degrees Fahrenheit".
A 100° reading on a Fahrenheit thermometer is equal to a 37° reading on a Celcius thermometer.
But "100 Fahrenheit degrees of temperature change" is equivalent "55 Celsius degrees of change".
That is an important distinction to understand the above temperature conversions.