determining the convertible debt discount rate

There isn't necessarily anything that informs the decision - recently, rates in the range of 20% have just become customary. Varying from the customary 20% may draw questions or concerns from investors.

Typically, startup companies have to lure in investors with some sort of incentive early on. The financial value of the premium rate has to be above 0% to really matter. However, it cannot be extremely high (above 30%), as that would cause too much dilution that drives down the value of the company's stock during the priced round.

My understanding is this custom of 20% evolved from the late 90's/early 00's (dot com boom) and is still accepted today. As stated above, varying from it may invite skepticism from investors.