While I can see how 20% is a bit high (I'm not complaining), I think offering a higher rate based on higher HP hurts the "little guy" who will generally earn less from HP. The people with higher HP are not as likely to need that high of an interest rate and are in a better position to earn with HP. Therefore, I would propose that any sliding scale on the interest rate go DOWN as the HP goes up. I would also base the scale on HP + HBD so people would not move HP to HDB (reducing their HP) to lower their base scale. That way, people with more HP would be encouraged to put that HP to use rather than let is sit in an interest bearing HBD savings account.