I want to clarify my previous comments about paying margin interest when your EFP stocks increase in value to the point where your cash balance goes negative. I commented that one should limit the frequency of adjustments to EFP positions, intended to return the cash balance to zero, so that the cost of the adjustments will be balanced by the benefits of the adjustments. The benefits of resetting a negative cash balance to zero involve avoiding the payment of roughly 1.5% interest on the negative cash balance. This rate is the net difference between the EFP interest you will earn on that negative cash balance, and the margin interest you will pay to the broker on that negative cash balance.
The figure of 1.5% is the relevant figure for purposes of balancing the cost of EFP portfolio adjustments against their benefits. The actual amount of margin interest which will be charged, and therefore lost, as a result of a negative cash balance, is much greater than this figure of 1.5%. It is currently about 6.8%. I don't have time to explain my math behind using these two different interest rates, for these two different purposes, so you'll have to form your own judgment about what you will do for your own account.
These figures of 1.5% and 6.8% step down to lower figures for larger negative cash balances, the largest of which will have rates of about 0.15% and (currently) 5.45%. See the IB website for details as to details on what IB charges for negative cash balances.
Keep in mind that in practice, the margin interest charged for negative cash balances generated by investing one's free cash in EFPs will be very small, at least if adjustments are performed when necessary, and the cost of such adjustments will also be very small.
Ways to help minimize the cost of margin interest on negative cash balances and adjustments include:
1) favor low-volatility stocks for your EFPs;
2) favor high-priced stocks because they require fewer EFPs and therefore less commissions to enter and to adjust;
3) diversify your EFP portfolio, preferring to use many different EFP stocks instead of just using one, because this will reduce the volatility of your overall EFP portfolio and thereby reduce negative cash balances, margin interest charges on them, and adjustments needed to reset them to zero.
Number 2) is a higher priority than number 3). You can figure out the reasoning behind this priority if you are good with probability math. I don't want to get into debates over it, so do whatever you want.
One possible approach, which I don't recommend, is to leave some free cash available, in order to cover any increase in your EFP stock values, so that you can prevent any negative cash balance generating margin interest charges. I recommend against this approach, because my calculations convince me that on the average, in the long run, you will lose out on earning EFP interest on the free cash, and this amount of interest not earned will be larger than the margin interest you will save by avoiding negative cash balances. I believe it is always best, in the long run, when entering or adjusting EFPs for the purpose of earning interest on free cash, to keep the cash balance as close as possible to zero, to ignore very small margin interest charges for negative balances, and to readjust if those interest charges are getting too big. I don't have time to explain my math, so again, you'll have to decide for yourself how to handle things.