How do I convert a character to a numeric value?

There is a general way of converting a character to its char number (which is equivalent to its unicode glyph slot in unicode), but it unfortunately doesn't work well in options to fontspec/unicode-math (an issue I'm aware of). Usually you can use something like this:

\newcount\foo
\foo=`\e

to assign the char number of "e" to \foo; this "backtick-escape" syntax works wherever TeX expects a number. However, it doesn't work if the argument is being expanded first, which is what happens in fontspec & unicode-math.

Now, you don't actually need the backslash in the code above; this will work too:

\foo=`e

The difference is that this cannot be used for "difficult" characters such as % and #. If you're just using letters, this won't be a problem.

So in unicode-math you should be able to write

range={`e,`j,...}

Or with whatever letters you need.