How much to increase the rent by for tenants partner to be added

[Marie-therese Tai]

My tenant wants to move in his pregnant girlfriend, what is an acceptable additional amount to charge ? . I havent raised the rent in the almost 3 years he has lived there , he currently pays $1K/mo for a large studio. I don't want to gouge him, it's Brooklyn , in an area that is slowly regentrifying, but rents are expensive and I suspect I'm actually below market rentwise