Bicycle tire questions. under inflate or over inflate?

i have a giant bolder bike. i put a 80cc 2-stroke engine on it so i average about 50kph all the time. im 230lbs. i got myself some kevlar tires and slime filled tubes(at wallmart i know im cheap) the tires are rated for 40-60psi. i race around 90 degree corners doin 50 and it is a mountain bike. to prevent flats should i inflate to 40 psi or 60 psi. also with the kevlar tires i got actualy do what they say on the box and prevent flats. because i ride very far away from home and i cant be getting flats. also i put the tire on with my hands. no tools. it was a fairly loose fit. ive riden about 30 kilometers on my new tire so far and it seems good. will it last. and how to i prevent flats. thanks