Here is the page on the MCPS webpage about substitute teacher pay, in case you didn't see it already:
https://www.montgomeryschoolsmd.org/departments/personnel/substitutes/info/
I used to know some people that subbed.
I don't know if much has changed but two things is that after you're there for a certain amount of hours per day, it's automatically counted as a full day. I'm just throwing numbers out but the way it was explained to me was if you're there for six hours, it'll get counted as eight hours regardless. But anything less then that is just for the hours of worked.
Then it helps to have the principal know and like you. Where principals can request who they want for substitutes and some people were happy that the principal had them in mind whenever a longterm sub opportunity comes up.