Hi Stan,
You need to buy a thermometer, and put it in your snake's basking spot. Then you'll know what temperature your snake is experiencing. The temp of the basking spot for garters should be in the upper 80s (~88F) during the warmest part of the day (don't know about Dekay's snakes, though). I think a 60watt bulb should get you in the ballpark. Rheostats are very nice because they let you decrease the wattage to get the right temperature.