1. Enter weight in the [Number1] field.
2. Automatically display the Duration that matches the weight in the [Duration1] field(using a formula).
3. Copy the [Duration1] field value to the [Duration] field(Copy everything at once using a macro).

If this process is what you were thinking of and you need more details, please email me.
ineejuatyahoodotcom

P.S.
Is the smallest unit of weight 1 ton? How many days is 1 ton?