calculation between two fields...HELP!



OK I have two numeric fields. One is WATTS and one is BTUH. When a number is entered in watts, I have a calculation of Watts * 3.41 to give the btuh. Works beautiful, the number shows up in the btuh field. But I also want to enter BTUH in that field and it calculate the watts. So in the watt field i have BTUH / 3.41. And it works only if you enter btuh first, it shows up in the watts, but if you put a watt in first and then try to enter a btuh it doesn't calculate in the watt. How do I fix this problem??? Please help me!!! Thanks for looking!