I need to create a variable labeled fc' to represent the compressive strength of concrete("f" and apostrophe being on the normal typing line, and "c" being treated as a subscript). From looking online I have come across round about ways of inputting similar variables using the Windows Character Map. Some characters are available in the character map that SMath will treat as a normal line of typing character, but the character displays as a subscript, such as U+2093: Latin Subscript Small letter X. Unfortunately, nothing is available for c.

Any help is appreciated.

Thanks

As for now, the simplest way is to use f'.c (that is widely used too).

I know ACI 318 has f.c^’, but there is not yet a way to freely mix baseline/subscript/superscript characters (also, superscript characters are only the ones available from the unicode tables).

... as you type f, you have 3 useful upper-script:
coma, Alt238, Alt248
and about 75 sub-script form AltKey