open air attenuation

i 'd like to ask about open air attenuation for a transmitter having out power of 6dbm,a receiver of sensitivity -110 dbm,also if there are obstacles such as walls or cars,all at 300Mhz frequency,please give me fomulas.
thanks

This is very complex. You cannot get better than the free space inverse square law. This is from building roofs to other building roofs. At your frequency and closer to the ground you will have ground wave propagation which has high loss. Metal objects like motor vehicles will cast shadows on the far side. Dielectric material will have reflection losses.

Your best course is to go to a typical location and see how far you can transmit. Compare this to the free space case.