There is a chart,
www.bmfa.org/handbook/hbook_13.html
You can do it without the chart. Take the first two digits after the decimal point and add 60. Thus 35.000 is 00 + 60 = channel number 60. 35.010 is 01+ 60 = channel 61. 35.220 is 22 +60 = channel 82.
For frequencies starting with 34, the result of adding 60 is over 100 so remove the 100 from the answer. Thus 34.950 is 95 + 60 = 155, remove the 1 to get channel 55.
Harry