Rent - around
*****
to
*****
/ mth considering your cats (proper, not modernized much)
Food - can live on 300 Baht/day (not being foodie, cook and sometimes go to local small shops) so 10,000 Baht/mth
Other cost like car fuel, go nearby places as short trips - 10,000 Baht/ mth (use smartly)
For your additional lifestyle cost - 10,000 Baht/ mth
Therefore, around 40,000 Baht per month is more than enough to stay in Chiang Mai. You can even reduce it to 25,000 Baht/ month.