It depends on your pricing, but a good guideline is to make the bid decrement somewhere between 0.25% and 0.5% of the item price. The front and back buffers are usually equal to each other and half of whatever you make the bid decrement (so 0.125% to 0.25%).
Suppliers often find it easier if you do these as an amount (as opposed to a percentage), but you can convert them over pretty easily. Here's an example.
Your starting price for an item is $143. That means your bid decrement should be between $0.36 (143 * 0.0025) and $0.73 (143 * .005). I would use $0.50. The front and back buffer would each be $0.25.
Hope that helps!
I would suggest using the percentage option because when you use the normal value option, it is a fixed value.
for eg: if a supplier starts with a price of $1000 and the bid decrement is $20 (2%) then when the supplier reduces his price from $1000 to $800, the bid decrement still remains $20 which is 3% of 800 which may be not possible for the supplier.
Whereas if you are using the percentage option than the supplier has to reduce the $16 which may still be possible.