I think that's as close as you're going to get. Beware that the whole thing is logarithmic, so 0.5 stops from F/8 to F/11 is not F/9.5, but more like F/9.
Yes, and its log base 2. Actually a simple solution to such problems as to do a little maths.
If any thing says: X + yStops
(y can also be a fraction)
Then the required aperture is: X * 2^(y/2)
So in this example: 2.8 + 5/10
= 2.8 * 2^(5/20)
= 2.8 * 2^(1/4)
= 2.8 * 1.1892
But we do not have 3.33 so go to 3.5, thats the closest you can get. BTW 3.5 is actually 2/3stop down 2.8.
Similarly, For 1.4 + 5/10 gives 1.66 which is not not available so the closest will be 1.8. Again 1.8 is 2/3 stop down 1.4.
You see the problem here is that the aperture quantization is done in thirds not in halfs. But, if you really want your exposure to be spot on. Try nutral density filters. these filters can be combined for all kinds of fractional