Do not use the expression a percentage or a proportion when what you mean is some, as in:
This drug has proved of much value in a percentage of cases.
The London Branch of the National Association of Fire Officers, which includes a proportion of station officers. . .
Here percentage and proportion pretend to mean something more than some, but do not really do so. They do not give the reader any idea of the number or proportion of the successful cases or station officers. One per cent is just as much "a percentage" as ninety-nine per cent.
Do not forget the simple words many, few, and some; and do not use percentage or proportion unless you want to express not an absolute number but the relation of one number to another, and can give at least an approximate degree of exactitude; so that though you may not be able to put an actual figure on the percentage or proportion, you can at any rate say "a high percentage", "a large proportion", "a low percentage", "a small proportion".
But fraction is different. It has become so common to use "only a fraction" in the sense of "only a small fraction" that it would be pedantry to object that 999/1000 is as much a fraction as 1/1000 just as it would certainly be pedantry to point out to anyone who says "He has got a temperature" that 98 degrees is just as much a temperature" as 104.