I'm not sure if using TX's rate is a proper way to go. Perhaps gun ownership rates are a better way to go. Use the license rate in a state with similiar ownership rates to that of MD. IIRC, license to carry rates range typically from 1% to about 6% in shall-issues. My guess is MD would probably be on the very low end, at least at the beginning. Other factors like National Reciprocity could drive those numbers up as well.
I have a restricted MD (it really does me no good I can't use it outside of work and never use it for work) that expires this year and wont get renewed so I put No MD 2 non res Utah & VA