Retract the dome so you get flat face readings.
One reading pointed straight at the source, yes in this case really straight at the sun.
The second reading is normally done pointed at the camera, do this one in the shade.
If the readings are as you say f/8 @ 500 And f/8 @ 60 you split the difference in stops. So if you adjust time you would try to get to about 1/180 (which is 1/125 and 1/2) or If you adjust f-stop f/4.5 (f/4 and 1/2).
This splits your losses evenly between highlight and shadow.
Give it a try on an experimental roll. If the shadows are good you are set to go, if you loose too much at one end or the other in a print adjust your EI.