+1 on the box speed.
And, the F5, like many other cameras today using matrix metering, are generally on the fat side of exposure. Most of my matrix metering cameras are calibrated for negative materials and will give you a transparency that is overexposed by 1/3 to 2/3 of a stop. With that in mind, shooting box speed will give you a neg that is fully exposed, no need to downrate the ISO again. My F4 runs about a 1/2 stop fat on exposure, on average.